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Section 2.1–2.2
   The Derivative and Rates of Change
      The Derivative as a Function

                     V63.0121.041, Calculus I

                          New York University


                       September 27, 2010


Announcements

   Quiz this week in recitation on §§1.1–1.4
   Get-to-know-you/photo due Friday October 1

                                                .   .   .   .   .   .
Announcements




         Quiz this week in recitation
         on §§1.1–1.4
         Get-to-know-you/photo
         due Friday October 1




                                                                   .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010       2 / 46
Format of written work

Please:
       Use scratch paper and
       copy your final work onto
       fresh paper.
       Use loose-leaf paper (not
       torn from a notebook).
       Write your name, lecture
       section, assignment
       number, recitation, and
       date at the top.
     Staple your homework
     together.
See the website for more information.

                                                                   .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010       3 / 46
Objectives for Section 2.1

         Understand and state the
         definition of the derivative
         of a function at a point.
         Given a function and a
         point in its domain, decide
         if the function is
         differentiable at the point
         and find the value of the
         derivative at that point.
         Understand and give
         several examples of
         derivatives modeling rates
         of change in science.

                                                                   .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010       4 / 46
Objectives for Section 2.2



         Given a function f, use the
         definition of the derivative
         to find the derivative
         function f’.
         Given a function, find its
         second derivative.
         Given the graph of a
         function, sketch the graph
         of its derivative.




                                                                   .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010       5 / 46
Outline

Rates of Change
   Tangent Lines
   Velocity
   Population growth
   Marginal costs

The derivative, defined
  Derivatives of (some) power functions
  What does f tell you about f′ ?

How can a function fail to be differentiable?

Other notations

The second derivative

                                                                   .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010       6 / 46
The tangent problem

Problem
Given a curve and a point on the curve, find the slope of the line
tangent to the curve at that point.




                                                                   .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010       7 / 46
The tangent problem

Problem
Given a curve and a point on the curve, find the slope of the line
tangent to the curve at that point.

Example
Find the slope of the line tangent to the curve y = x2 at the point (2, 4).




                                                                   .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010       7 / 46
Graphically and numerically

          y
          .
                                                                                         x2 − 22
                                                                       x           m=
                                                                                          x−2




       . .
       4                          .




           .                    .         x
                                          .
                              2
                              .
                                                                           .   .     .      .      .     .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative                September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                               x2 − 22
                                                                             x           m=
                                                                                                x−2
                                                                             3
       . .
       9                               .




       . .
       4                          .




           .                    .       .       x
                                                .
                              2
                              .       3
                                      .
                                                                                 .   .     .      .      .     .

 V63.0121.041, Calculus I (NYU)             Section 2.1–2.2 The Derivative                September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                                  x2 − 22
                                                                             x           m=
                                                                                                   x−2
                                                                             3           5
       . .
       9                               .




       . .
       4                          .




           .                    .       .       x
                                                .
                              2
                              .       3
                                      .
                                                                                 .   .        .      .      .     .

 V63.0121.041, Calculus I (NYU)             Section 2.1–2.2 The Derivative                   September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                                x2 − 22
                                                                           x           m=
                                                                                                 x−2
                                                                           3           5
                                                                           2.5


 . .25 .
 6                                    .


       . .
       4                          .




           .                   . .            x
                                              .
                              22
                              . . .5
                                                                               .   .        .      .      .     .

 V63.0121.041, Calculus I (NYU)           Section 2.1–2.2 The Derivative                   September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                                  x2 − 22
                                                                           x           m=
                                                                                                   x−2
                                                                           3           5
                                                                           2.5         4.5


 . .25 .
 6                                    .


       . .
       4                          .




           .                   . .            x
                                              .
                              22
                              . . .5
                                                                               .   .          .      .      .     .

 V63.0121.041, Calculus I (NYU)           Section 2.1–2.2 The Derivative                     September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                               x2 − 22
                                                                        x           m=
                                                                                                x−2
                                                                        3           5
                                                                        2.5         4.5
                                                                        2.1



 . .41 .
 4                                 .
     . .
     4                            .




           .                   ..          x
                                           .
                             2
                             .. .1
                              2
                                                                            .   .          .      .      .     .

 V63.0121.041, Calculus I (NYU)        Section 2.1–2.2 The Derivative                     September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                               x2 − 22
                                                                        x           m=
                                                                                                x−2
                                                                        3           5
                                                                        2.5         4.5
                                                                        2.1         4.1



 . .41 .
 4                                 .
     . .
     4                            .




           .                   ..          x
                                           .
                             2
                             .. .1
                              2
                                                                            .   .          .      .      .     .

 V63.0121.041, Calculus I (NYU)        Section 2.1–2.2 The Derivative                     September 27, 2010       8 / 46
Graphically and numerically

           y
           .
                                                                                          x2 − 22
                                                                        x           m=
                                                                                           x−2
                                                                        3    5
                                                                        2.5  4.5
                                                                        2.1  4.1
                                                                        2.01


. .0401 .
4     4
      .                            .




            .                  .           x
                                           .
                            2.
                            . .01
                              2
                                                                            .   .     .      .      .     .

  V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative                September 27, 2010       8 / 46
Graphically and numerically

           y
           .
                                                                                            x2 − 22
                                                                        x           m=
                                                                                             x−2
                                                                        3           5
                                                                        2.5         4.5
                                                                        2.1         4.1
                                                                        2.01        4.01


. .0401 .
4     4
      .                            .




            .                  .           x
                                           .
                            2.
                            . .01
                              2
                                                                            .   .       .      .      .     .

  V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative                  September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                           x2 − 22
                                                                       x           m=
                                                                                            x−2
                                                                       3           5
                                                                       2.5         4.5
                                                                       2.1         4.1
                                                                       2.01        4.01


       . .
       4                          .


                                                                       1
       . .
       1              .
         .            .         .         x
                                          .
                    1
                    .         2
                              .
                                                                           .   .       .      .      .     .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative                  September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                            x2 − 22
                                                                       x           m=
                                                                                             x−2
                                                                       3           5
                                                                       2.5         4.5
                                                                       2.1         4.1
                                                                       2.01        4.01


       . .
       4                          .


                                                                       1           3
       . .
       1              .
         .            .         .         x
                                          .
                    1
                    .         2
                              .
                                                                           .   .        .      .      .     .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative                   September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                            x2 − 22
                                                                       x           m=
                                                                                             x−2
                                                                       3           5
                                                                       2.5         4.5
                                                                       2.1         4.1
                                                                       2.01        4.01


       . .
       4                          .
                                                                       1.5
 . .25 .
 2                        .
                                                                       1           3
           .             . .              x
                                          .
                       1 2
                       . .5 .
                                                                           .   .        .      .      .     .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative                   September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                              x2 − 22
                                                                       x           m=
                                                                                               x−2
                                                                       3           5
                                                                       2.5         4.5
                                                                       2.1         4.1
                                                                       2.01        4.01


       . .
       4                          .
                                                                       1.5         3.5
 . .25 .
 2                        .
                                                                       1           3
           .             . .              x
                                          .
                       1 2
                       . .5 .
                                                                           .   .          .      .      .     .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative                     September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                           x2 − 22
                                                                    x           m=
                                                                                            x−2
                                                                    3           5
                                                                    2.5         4.5
                                                                    2.1         4.1
                                                                    2.01        4.01


     . .
     4                         .
 . .61 .
 3                            .                                     1.9
                                                                    1.5         3.5
                                                                    1           3
           .                 ..        x
                                       .
                           1.
                           . .9
                             2
                                                                        .   .          .      .      .     .

 V63.0121.041, Calculus I (NYU)    Section 2.1–2.2 The Derivative                     September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                           x2 − 22
                                                                    x           m=
                                                                                            x−2
                                                                    3           5
                                                                    2.5         4.5
                                                                    2.1         4.1
                                                                    2.01        4.01


     . .
     4                         .
 . .61 .
 3                            .                                     1.9         3.9
                                                                    1.5         3.5
                                                                    1           3
           .                 ..        x
                                       .
                           1.
                           . .9
                             2
                                                                        .   .          .      .      .     .

 V63.0121.041, Calculus I (NYU)    Section 2.1–2.2 The Derivative                     September 27, 2010       8 / 46
Graphically and numerically

           y
           .
                                                                                            x2 − 22
                                                                        x           m=
                                                                                             x−2
                                                                        3           5
                                                                        2.5         4.5
                                                                        2.1         4.1
                                                                        2.01        4.01

                                                                        1.99
. .9601 .
3     4
      .                            .
                                                                        1.9  3.9
                                                                        1.5  3.5
                                                                        1    3
            .                  .           x
                                           .
                            1.
                            . .99
                               2
                                                                            .   .       .      .      .     .

  V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative                  September 27, 2010       8 / 46
Graphically and numerically

           y
           .
                                                                                            x2 − 22
                                                                        x           m=
                                                                                             x−2
                                                                        3           5
                                                                        2.5         4.5
                                                                        2.1         4.1
                                                                        2.01        4.01

                                                                        1.99        3.99
. .9601 .
3     4
      .                            .
                                                                        1.9         3.9
                                                                        1.5         3.5
                                                                        1           3
            .                  .           x
                                           .
                            1.
                            . .99
                               2
                                                                            .   .       .      .      .     .

  V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative                  September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                           x2 − 22
                                                                       x           m=
                                                                                            x−2
                                                                       3           5
                                                                       2.5         4.5
                                                                       2.1         4.1
                                                                       2.01        4.01

                                                                       1.99        3.99
       . .
       4                          .
                                                                       1.9         3.9
                                                                       1.5         3.5
                                                                       1           3
           .                    .         x
                                          .
                              2
                              .
                                                                           .   .       .      .      .     .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative                  September 27, 2010       8 / 46
Graphically and numerically

           y
           .
                                                                                                   x2 − 22
                                                                               x           m=
                                                                                                    x−2
                                                                               3           5
        . .
        9                                 .
                                                                               2.5         4.5
                                                                               2.1         4.1
                                                                               2.01        4.01
   . .25 .
   6                                  .
                                                                               limit
    . .41 .
    4                             .                                            1.99        3.99
. .0401 .
4.9601 .
3 . .61
    3
        4
        .                      ..                                              1.9         3.9
                                                                               1.5         3.5
   . .25 .
   2                       .
                                                                               1           3
       . .
       1              .
         .            . . ... . .                 x
                                                  .
                     1 1 . .. .1 3
                     . . .5 .99 .5 .
                          12 .
                          2.9 2
                             2
                            .01
                                                                                   .   .       .      .      .     .

  V63.0121.041, Calculus I (NYU)              Section 2.1–2.2 The Derivative                  September 27, 2010       8 / 46
Graphically and numerically

          y
          .
                                                                                         x2 − 22
                                                                       x           m=
                                                                                          x−2
                                                                       3    5
                                                                       2.5  4.5
                                                                       2.1  4.1
                                                                       2.01 4.01
                                                                            4
                                                                       1.99 3.99
       . .
       4                          .
                                                                       1.9  3.9
                                                                       1.5  3.5
                                                                       1    3
           .                    .         x
                                          .
                              2
                              .
                                                                           .   .     .      .      .     .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative                September 27, 2010       8 / 46
The tangent problem

Problem
Given a curve and a point on the curve, find the slope of the line
tangent to the curve at that point.

Example
Find the slope of the line tangent to the curve y = x2 at the point (2, 4).

Upshot
If the curve is given by y = f(x), and the point on the curve is (a, f(a)),
then the slope of the tangent line is given by

                                                       f(x) − f(a)
                                  mtangent = lim
                                                x→a       x−a

                                                                      .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010       9 / 46
Velocity
.
Problem
Given the position function of a moving object, find the velocity of the object at
a certain instant in time.

Example
Drop a ball off the roof of the Silver Center so that its height can be described
by
                                   h(t) = 50 − 5t2
where t is seconds after dropping it and h is meters above the ground. How
fast is it falling one second after we drop it?




.                                                                     .   .     .      .      .    .

    V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   10 / 46
Numerical evidence



                                        h(t) = 50 − 5t2
Fill in the table:
                                                        h(t) − h(1)
                                  t         vave =
                                                           t−1
                                  2         − 15




                                                                       .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative           September 27, 2010   11 / 46
Numerical evidence



                                          h(t) = 50 − 5t2
Fill in the table:
                                                          h(t) − h(1)
                                  t           vave =
                                                             t−1
                                  2           − 15
                                  1.5




                                                                         .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)         Section 2.1–2.2 The Derivative           September 27, 2010   11 / 46
Numerical evidence



                                          h(t) = 50 − 5t2
Fill in the table:
                                                          h(t) − h(1)
                                  t           vave =
                                                             t−1
                                  2           − 15
                                  1.5         − 12.5




                                                                         .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)         Section 2.1–2.2 The Derivative           September 27, 2010   11 / 46
Numerical evidence



                                          h(t) = 50 − 5t2
Fill in the table:
                                                          h(t) − h(1)
                                  t           vave =
                                                             t−1
                                  2           − 15
                                  1.5         − 12.5
                                  1.1




                                                                         .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)         Section 2.1–2.2 The Derivative           September 27, 2010   11 / 46
Numerical evidence



                                          h(t) = 50 − 5t2
Fill in the table:
                                                          h(t) − h(1)
                                  t           vave =
                                                             t−1
                                  2           − 15
                                  1.5         − 12.5
                                  1.1         − 10.5




                                                                         .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)         Section 2.1–2.2 The Derivative           September 27, 2010   11 / 46
Numerical evidence



                                         h(t) = 50 − 5t2
Fill in the table:
                                                        h(t) − h(1)
                                  t         vave =
                                                           t−1
                                  2         − 15
                                  1.5       − 12.5
                                  1.1       − 10.5
                                  1.01




                                                                       .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative           September 27, 2010   11 / 46
Numerical evidence



                                         h(t) = 50 − 5t2
Fill in the table:
                                                        h(t) − h(1)
                                  t         vave =
                                                           t−1
                                  2         − 15
                                  1.5       − 12.5
                                  1.1       − 10.5
                                  1.01      − 10.05




                                                                       .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative           September 27, 2010   11 / 46
Numerical evidence



                                        h(t) = 50 − 5t2
Fill in the table:
                                                        h(t) − h(1)
                                  t         vave =
                                                           t−1
                                  2         − 15
                                  1.5       − 12.5
                                  1.1       − 10.5
                                  1.01      − 10.05
                                  1.001




                                                                       .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative           September 27, 2010   11 / 46
Numerical evidence



                                        h(t) = 50 − 5t2
Fill in the table:
                                                        h(t) − h(1)
                                  t         vave =
                                                           t−1
                                  2         − 15
                                  1.5       − 12.5
                                  1.1       − 10.5
                                  1.01      − 10.05
                                  1.001     − 10.005




                                                                       .   .    .      .      .     .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative           September 27, 2010   11 / 46
Velocity
.
Problem
Given the position function of a moving object, find the velocity of the object at
a certain instant in time.

Example
Drop a ball off the roof of the Silver Center so that its height can be described
by
                                   h(t) = 50 − 5t2
where t is seconds after dropping it and h is meters above the ground. How
fast is it falling one second after we drop it?

Solution
The answer is

                      (50 − 5t2 ) − 45        5 − 5t2       5(1 − t)(1 + t)
               v = lim                 = lim          = lim
                     t→1     t−1           t→1 t − 1    t→1     t−1
                 = (−5) lim (1 + t) = −5 · 2 = −10
                            t→1

.                                                                      .   .     .      .      .    .

    V63.0121.041, Calculus I (NYU)    Section 2.1–2.2 The Derivative           September 27, 2010   12 / 46
Velocity in general


                                                                             . . = h(t)
                                                                               y
Upshot
                                                                   . (t0 ) .
                                                                   h                     .
If the height function is given by
h(t), the instantaneous velocity                                                 . h
                                                                                 ∆
at time t0 is given by
                                                         . (t0 + ∆t) .
                                                         h                                       .
            h(t) − h(t0 )
   v = lim
       t→t0    t − t0
             h(t0 + ∆t) − h(t0 )
      = lim
       ∆t→0           ∆t
                                                                             .            . . t .
                                                                                            ∆          ..
                                                                                                        t
                                                                                        t
                                                                                        .0      t
                                                                                                .

                                                                    .    .          .        .    .         .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative                  September 27, 2010    13 / 46
Population growth
Problem
Given the population function of a group of organisms, find the rate of
growth of the population at a particular instant.




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   14 / 46
Population growth
Problem
Given the population function of a group of organisms, find the rate of
growth of the population at a particular instant.

Example
Suppose the population of fish in the East River is given by the function

                                                  3et
                                     P(t) =
                                                 1 + et
where t is in years since 2000 and P is in millions of fish. Is the fish
population growing fastest in 1990, 2000, or 2010? (Estimate
numerically)



                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   14 / 46
Derivation


Solution
Let ∆t be an increment in time and ∆P the corresponding change in
population:
                        ∆P = P(t + ∆t) − P(t)
This depends on ∆t, so ideally we would want
                                  (                  )
                  ∆P           1     3et+∆t    3et
              lim     = lim                  −
             ∆t→0 ∆t     ∆t→0 ∆t    1 + et+∆t 1 + et




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   15 / 46
Derivation


Solution
Let ∆t be an increment in time and ∆P the corresponding change in
population:
                        ∆P = P(t + ∆t) − P(t)
This depends on ∆t, so ideally we would want
                                  (                  )
                  ∆P           1     3et+∆t    3et
              lim     = lim                  −
             ∆t→0 ∆t     ∆t→0 ∆t    1 + et+∆t 1 + et

But rather than compute a complicated limit analytically, let us
approximate numerically. We will try a small ∆t, for instance 0.1.



                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   15 / 46
Numerical evidence
Solution (Continued)
To approximate the population change in year n, use the difference
         P(t + ∆t) − P(t)
quotient                  , where ∆t = 0.1 and t = n − 2000.
               ∆t


    r1990



    r2000



    r2010


                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   16 / 46
Numerical evidence
Solution (Continued)
To approximate the population change in year n, use the difference
         P(t + ∆t) − P(t)
quotient                  , where ∆t = 0.1 and t = n − 2000.
               ∆t

                 P(−10 + 0.1) − P(−10)
    r1990 ≈
                          0.1


                 P(0.1) − P(0)
    r2000 ≈
                      0.1


                 P(10 + 0.1) − P(10)
    r2010 ≈
                         0.1

                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   16 / 46
Numerical evidence
Solution (Continued)
To approximate the population change in year n, use the difference
         P(t + ∆t) − P(t)
quotient                  , where ∆t = 0.1 and t = n − 2000.
               ∆t
                                                            (                                         )
               P(−10 + 0.1) − P(−10)    1                           3e−9.9    3e−10
    r1990    ≈                       =                                      −
                        0.1            0.1                         1 + e−9.9 1 + e−10

                                           (                                 )
                 P(0.1) − P(0)    1             3e0.1    3e0
    r2000 ≈                    =                       −
                      0.1        0.1           1 + e0.1 1 + e0

                                                      (                                       )
               P(10 + 0.1) − P(10)    1                    3e10.1    3e10
    r2010    ≈                     =                               −
                       0.1           0.1                  1 + e10.1 1 + e10

                                                                     .   .         .      .       .       .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative                 September 27, 2010       16 / 46
Numerical evidence
Solution (Continued)
To approximate the population change in year n, use the difference
         P(t + ∆t) − P(t)
quotient                  , where ∆t = 0.1 and t = n − 2000.
               ∆t
                                           (                   )
               P(−10 + 0.1) − P(−10)     1   3e−9.9    3e−10
    r1990    ≈                       =               −
                         0.1            0.1 1 + e−9.9 1 + e−10
             = 0.000143229
                                  (                )
               P(0.1) − P(0)    1    3e0.1    3e0
    r2000    ≈               =             −
                    0.1        0.1 1 + e0.1 1 + e0

                                                      (                                 )
               P(10 + 0.1) − P(10)    1                    3e10.1    3e10
    r2010    ≈                     =                               −
                       0.1           0.1                  1 + e10.1 1 + e10

                                                                   .   .     .      .       .   .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   16 / 46
Numerical evidence
Solution (Continued)
To approximate the population change in year n, use the difference
         P(t + ∆t) − P(t)
quotient                  , where ∆t = 0.1 and t = n − 2000.
               ∆t
                                            (                    )
               P(−10 + 0.1) − P(−10)     1     3e−9.9      3e−10
    r1990    ≈                        =                −
                          0.1           0.1 1 + e−9.9 1 + e−10
             = 0.000143229
                                   (                 )
               P(0.1) − P(0)     1   3e0.1      3e0
    r2000    ≈                =             −
                    0.1         0.1 1 + e0.1 1 + e0
             = 0.749376
                                        (                    )
               P(10 + 0.1) − P(10)    1     3e10.1     3e10
    r2010    ≈                     =                −
                        0.1          0.1 1 + e10.1 1 + e10

                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   16 / 46
Numerical evidence
Solution (Continued)
To approximate the population change in year n, use the difference
         P(t + ∆t) − P(t)
quotient                  , where ∆t = 0.1 and t = n − 2000.
               ∆t
                                            (                    )
               P(−10 + 0.1) − P(−10)     1     3e−9.9      3e−10
    r1990    ≈                        =                −
                          0.1           0.1 1 + e−9.9 1 + e−10
             = 0.000143229
                                   (                 )
               P(0.1) − P(0)     1   3e0.1      3e0
    r2000    ≈                =             −
                    0.1         0.1 1 + e0.1 1 + e0
             = 0.749376
                                        (                    )
               P(10 + 0.1) − P(10)    1     3e10.1     3e10
    r2010    ≈                     =                −
                        0.1          0.1 1 + e10.1 1 + e10
             = 0.0001296
                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   16 / 46
Population growth
.
Problem
Given the population function of a group of organisms, find the rate of growth
of the population at a particular instant.

Example
Suppose the population of fish in the East River is given by the function

                                                     3et
                                         P(t) =
                                                    1 + et
where t is in years since 2000 and P is in millions of fish. Is the fish population
growing fastest in 1990, 2000, or 2010? (Estimate numerically)

Answer
We estimate the rates of growth to be 0.000143229, 0.749376, and 0.0001296.
So the population is growing fastest in 2000.
.
                                                                      .   .     .      .      .    .

    V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   17 / 46
Population growth in general




Upshot
The instantaneous population growth is given by

                                       P(t + ∆t) − P(t)
                                  lim
                                  ∆t→0       ∆t




                                                                    .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)    Section 2.1–2.2 The Derivative           September 27, 2010   18 / 46
Marginal costs

Problem
Given the production cost of a good, find the marginal cost of
production after having produced a certain quantity.




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   19 / 46
Marginal costs

Problem
Given the production cost of a good, find the marginal cost of
production after having produced a certain quantity.

Example
Suppose the cost of producing q tons of rice on our paddy in a year is

                                  C(q) = q3 − 12q2 + 60q

We are currently producing 5 tons a year. Should we change that?




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   19 / 46
Comparisons



Solution

                                  C(q) = q3 − 12q2 + 60q
Fill in the table:

               q C(q)
               4
               5
               6




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   20 / 46
Comparisons



Solution

                                  C(q) = q3 − 12q2 + 60q
Fill in the table:

               q C(q)
               4 112
               5
               6




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   20 / 46
Comparisons



Solution

                                  C(q) = q3 − 12q2 + 60q
Fill in the table:

               q C(q)
               4 112
               5 125
               6




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   20 / 46
Comparisons



Solution

                                  C(q) = q3 − 12q2 + 60q
Fill in the table:

               q C(q)
               4 112
               5 125
               6 144




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   20 / 46
Comparisons



Solution

                                  C(q) = q3 − 12q2 + 60q
Fill in the table:

               q C(q) AC(q) = C(q)/q
               4 112
               5 125
               6 144




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   20 / 46
Comparisons



Solution

                                  C(q) = q3 − 12q2 + 60q
Fill in the table:

               q C(q) AC(q) = C(q)/q
               4 112        28
               5 125
               6 144




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   20 / 46
Comparisons



Solution

                                  C(q) = q3 − 12q2 + 60q
Fill in the table:

               q C(q) AC(q) = C(q)/q
               4 112        28
               5 125        25
               6 144




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   20 / 46
Comparisons



Solution

                                  C(q) = q3 − 12q2 + 60q
Fill in the table:

               q C(q) AC(q) = C(q)/q
               4 112        28
               5 125        25
               6 144        24




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   20 / 46
Comparisons



Solution

                                  C(q) = q3 − 12q2 + 60q
Fill in the table:

               q C(q) AC(q) = C(q)/q ∆C = C(q + 1) − C(q)
               4 112        28
               5 125        25
               6 144        24




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   20 / 46
Comparisons



Solution

                                  C(q) = q3 − 12q2 + 60q
Fill in the table:

               q C(q) AC(q) = C(q)/q ∆C = C(q + 1) − C(q)
               4 112        28               13
               5 125        25
               6 144        24




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   20 / 46
Comparisons



Solution

                                  C(q) = q3 − 12q2 + 60q
Fill in the table:

               q C(q) AC(q) = C(q)/q ∆C = C(q + 1) − C(q)
               4 112        28               13
               5 125        25               19
               6 144        24




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   20 / 46
Comparisons



Solution

                                  C(q) = q3 − 12q2 + 60q
Fill in the table:

               q C(q) AC(q) = C(q)/q ∆C = C(q + 1) − C(q)
               4 112        28               13
               5 125        25               19
               6 144        24               31




                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   20 / 46
Marginal costs

Problem
Given the production cost of a good, find the marginal cost of
production after having produced a certain quantity.

Example
Suppose the cost of producing q tons of rice on our paddy in a year is

                                  C(q) = q3 − 12q2 + 60q

We are currently producing 5 tons a year. Should we change that?

Answer
If q = 5, then C = 125, ∆C = 19, while AC = 25. So we should
produce more to lower average costs.
                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   21 / 46
Marginal Cost in General


Upshot

      The incremental cost

                                  ∆C = C(q + 1) − C(q)

      is useful, but is still only an average rate of change.




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   22 / 46
Marginal Cost in General


Upshot

      The incremental cost

                                    ∆C = C(q + 1) − C(q)

      is useful, but is still only an average rate of change.
      The marginal cost after producing q given by

                                             C(q + ∆q) − C(q)
                                  MC = lim
                                        ∆q→0       ∆q

      is more useful since it’s an instantaneous rate of change.


                                                                      .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)      Section 2.1–2.2 The Derivative           September 27, 2010   22 / 46
Outline

Rates of Change
   Tangent Lines
   Velocity
   Population growth
   Marginal costs

The derivative, defined
  Derivatives of (some) power functions
  What does f tell you about f′ ?

How can a function fail to be differentiable?

Other notations

The second derivative

                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   23 / 46
The definition



All of these rates of change are found the same way!




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   24 / 46
The definition



All of these rates of change are found the same way!
Definition
Let f be a function and a a point in the domain of f. If the limit

                                      f(a + h) − f(a)       f(x) − f(a)
                     f′ (a) = lim                     = lim
                                  h→0        h          x→a    x−a

exists, the function is said to be differentiable at a and f′ (a) is the
derivative of f at a.




                                                                        .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)        Section 2.1–2.2 The Derivative           September 27, 2010   24 / 46
Derivative of the squaring function

Example
Suppose f(x) = x2 . Use the definition of derivative to find f′ (a).




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   25 / 46
Derivative of the squaring function

Example
Suppose f(x) = x2 . Use the definition of derivative to find f′ (a).

Solution


                                f(a + h) − f(a)       (a + h)2 − a2
                 f′ (a) = lim                   = lim
                           h→0         h          h→0       h
                                (a2 + 2ah + h2 ) − a2        2ah + h2
                         = lim                        = lim
                           h→0            h             h→0     h
                         = lim (2a + h) = 2a.
                             h→0




                                                                    .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)    Section 2.1–2.2 The Derivative           September 27, 2010   25 / 46
Derivative of the reciprocal function

Example
                  1
Suppose f(x) =      . Use the
                  x
definition of the derivative to
find f′ (2).




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   26 / 46
Derivative of the reciprocal function

Example
                  1
Suppose f(x) =      . Use the
                  x
definition of the derivative to
find f′ (2).                                                               x
                                                                           .

Solution


                     1/x − 1/2
       f′ (2) = lim                                                                        .
                   x→2 x−2                                             .                                x
                                                                                                        .
                       2−x
               = lim
                 x→2 2x(x − 2)
                     −1      1
               = lim     =−
                 x→2 2x      4
                                                                   .           .     .      .      .        .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative                   September 27, 2010       26 / 46
The Sure-Fire Sally Rule (SFSR) for adding Fractions
In anticipation of the question, “How did you get that?"




             a c  ad ± bc
              ± =
             b d    bd
 So
           1 1   2−x
            −
           x 2 = 2x
           x−2   x−2
                   2−x
               =
                 2x(x − 2)




                                                                    .   .     .      .      .    .

  V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   27 / 46
The Sure-Fire Sally Rule (SFSR) for adding Fractions
In anticipation of the question, “How did you get that?"




             a c  ad ± bc
              ± =
             b d    bd
 So
           1 1   2−x
            −
           x 2 = 2x
           x−2   x−2
                   2−x
               =
                 2x(x − 2)




                                                                    .   .     .      .      .    .

  V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   27 / 46
What does f tell you about f′ ?




      If f is a function, we can compute the derivative f′ (x) at each point
      x where f is differentiable, and come up with another function, the
      derivative function.
      What can we say about this function f′ ?




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   28 / 46
What does f tell you about f′ ?




      If f is a function, we can compute the derivative f′ (x) at each point
      x where f is differentiable, and come up with another function, the
      derivative function.
      What can we say about this function f′ ?
             If f is decreasing on an interval, f′ is negative (technically,
             nonpositive) on that interval




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   28 / 46
Derivative of the reciprocal function

Example
                  1
Suppose f(x) =      . Use the
                  x
definition of the derivative to
find f′ (2).                                                               x
                                                                           .

Solution


                     1/x − 1/2
       f′ (2) = lim                                                                        .
                   x→2 x−2                                             .                                x
                                                                                                        .
                       2−x
               = lim
                 x→2 2x(x − 2)
                     −1      1
               = lim     =−
                 x→2 2x      4
                                                                   .           .     .      .      .        .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative                   September 27, 2010       29 / 46
What does f tell you about f′ ?




      If f is a function, we can compute the derivative f′ (x) at each point
      x where f is differentiable, and come up with another function, the
      derivative function.
      What can we say about this function f′ ?
             If f is decreasing on an interval, f′ is negative (technically,
             nonpositive) on that interval
             If f is increasing on an interval, f′ is positive (technically,
             nonnegative) on that interval




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   30 / 46
Graphically and numerically

          y
          .
                                                                                           x2 − 22
                                                                       x           m=
                                                                                            x−2
                                                                       3           5
                                                                       2.5         4.5
                                                                       2.1         4.1
                                                                       2.01        4.01


       . .
       4                          .


                                                                       1
       . .
       1              .
         .            .         .         x
                                          .
                    1
                    .         2
                              .
                                                                           .   .       .      .      .    .

 V63.0121.041, Calculus I (NYU)       Section 2.1–2.2 The Derivative                 September 27, 2010   31 / 46
What does f tell you about f′ ?
.
Fact
If f is decreasing on (a, b), then f′ ≤ 0 on (a, b).




.                                                                     .   .     .      .      .    .

    V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   32 / 46
What does f tell you about f′ ?
.
Fact
If f is decreasing on (a, b), then f′ ≤ 0 on (a, b).

Proof.
If f is decreasing on (a, b), and ∆x > 0, then

                                                   f(x + ∆x) − f(x)
                           f(x + ∆x) < f(x) =⇒                      <0
                                                         ∆x




.                                                                       .   .     .      .      .    .

    V63.0121.041, Calculus I (NYU)     Section 2.1–2.2 The Derivative           September 27, 2010   32 / 46
What does f tell you about f′ ?
.
Fact
If f is decreasing on (a, b), then f′ ≤ 0 on (a, b).

Proof.
If f is decreasing on (a, b), and ∆x > 0, then

                                                   f(x + ∆x) − f(x)
                           f(x + ∆x) < f(x) =⇒                      <0
                                                         ∆x
But if ∆x < 0, then x + ∆x < x, and
                                                   f(x + ∆x) − f(x)
                           f(x + ∆x) > f(x) =⇒                      <0
                                                         ∆x

still!




.                                                                       .   .     .      .      .    .

    V63.0121.041, Calculus I (NYU)     Section 2.1–2.2 The Derivative           September 27, 2010   32 / 46
What does f tell you about f′ ?
.
Fact
If f is decreasing on (a, b), then f′ ≤ 0 on (a, b).

Proof.
If f is decreasing on (a, b), and ∆x > 0, then

                                                      f(x + ∆x) − f(x)
                           f(x + ∆x) < f(x) =⇒                         <0
                                                            ∆x
But if ∆x < 0, then x + ∆x < x, and
                                                      f(x + ∆x) − f(x)
                           f(x + ∆x) > f(x) =⇒                         <0
                                                            ∆x
                         f(x + ∆x) − f(x)
still! Either way,                        < 0, so
                               ∆x
                                                f(x + ∆x) − f(x)
                                 f′ (x) = lim                    ≤0
                                        ∆x→0          ∆x

.                                                                          .   .     .      .      .    .

    V63.0121.041, Calculus I (NYU)        Section 2.1–2.2 The Derivative           September 27, 2010   32 / 46
Outline

Rates of Change
   Tangent Lines
   Velocity
   Population growth
   Marginal costs

The derivative, defined
  Derivatives of (some) power functions
  What does f tell you about f′ ?

How can a function fail to be differentiable?

Other notations

The second derivative

                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   33 / 46
Differentiability is super-continuity

Theorem
If f is differentiable at a, then f is continuous at a.




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   34 / 46
Differentiability is super-continuity

Theorem
If f is differentiable at a, then f is continuous at a.

Proof.
We have
                                             f(x) − f(a)
                     lim (f(x) − f(a)) = lim             · (x − a)
                    x→a                 x→a      x−a
                                             f(x) − f(a)
                                      = lim              · lim (x − a)
                                        x→a      x−a       x→a
                                         ′
                                      = f (a) · 0 = 0




                                                                    .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)    Section 2.1–2.2 The Derivative           September 27, 2010   34 / 46
Differentiability is super-continuity

Theorem
If f is differentiable at a, then f is continuous at a.

Proof.
We have
                                             f(x) − f(a)
                     lim (f(x) − f(a)) = lim             · (x − a)
                    x→a                 x→a      x−a
                                             f(x) − f(a)
                                      = lim              · lim (x − a)
                                        x→a      x−a       x→a
                                         ′
                                      = f (a) · 0 = 0



Note the proper use of the limit law: if the factors each have a limit at
a, the limit of the product is the product of the limits.
                                                                    .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)    Section 2.1–2.2 The Derivative           September 27, 2010   34 / 46
Differentiability FAIL
Kinks


Example
Let f have the graph on the left-hand side below. Sketch the graph of
the derivative f′ .
                       f
                       .(x)




                          .       x
                                  .




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   35 / 46
Differentiability FAIL
Kinks


Example
Let f have the graph on the left-hand side below. Sketch the graph of
the derivative f′ .
                       f
                       .(x)                                                .′ (x)
                                                                           f




                          .       x
                                  .                                           .                  x
                                                                                                 .




                                                                   .   .          .   .      .       .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative             September 27, 2010      35 / 46
Differentiability FAIL
Kinks


Example
Let f have the graph on the left-hand side below. Sketch the graph of
the derivative f′ .
                       f
                       .(x)                                                .′ (x)
                                                                           f


                                                                              .

                          .       x
                                  .                                           .                  x
                                                                                                 .




                                                                   .   .          .   .      .       .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative             September 27, 2010      35 / 46
Differentiability FAIL
Cusps


Example
Let f have the graph on the left-hand side below. Sketch the graph of
the derivative f′ .
                       f
                       .(x)




                          .       x
                                  .




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   36 / 46
Differentiability FAIL
Cusps


Example
Let f have the graph on the left-hand side below. Sketch the graph of
the derivative f′ .
                       f
                       .(x)                                                .′ (x)
                                                                           f




                          .       x
                                  .                                           .                  x
                                                                                                 .




                                                                   .   .          .   .      .       .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative             September 27, 2010      36 / 46
Differentiability FAIL
Cusps


Example
Let f have the graph on the left-hand side below. Sketch the graph of
the derivative f′ .
                       f
                       .(x)                                                .′ (x)
                                                                           f




                          .       x
                                  .                                           .                  x
                                                                                                 .




                                                                   .   .          .   .      .       .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative             September 27, 2010      36 / 46
Differentiability FAIL
Cusps


Example
Let f have the graph on the left-hand side below. Sketch the graph of
the derivative f′ .
                       f
                       .(x)                                                .′ (x)
                                                                           f




                          .       x
                                  .                                           .                  x
                                                                                                 .




                                                                   .   .          .   .      .       .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative             September 27, 2010      36 / 46
Differentiability FAIL
Vertical Tangents


Example
Let f have the graph on the left-hand side below. Sketch the graph of
the derivative f′ .
                        f
                        .(x)




                           .       x
                                   .




                                                                    .   .     .      .      .    .

  V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   37 / 46
Differentiability FAIL
Vertical Tangents


Example
Let f have the graph on the left-hand side below. Sketch the graph of
the derivative f′ .
                        f
                        .(x)                                                .′ (x)
                                                                            f




                           .       x
                                   .                                           .                  x
                                                                                                  .




                                                                    .   .          .   .      .       .

  V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative             September 27, 2010      37 / 46
Differentiability FAIL
Vertical Tangents


Example
Let f have the graph on the left-hand side below. Sketch the graph of
the derivative f′ .
                        f
                        .(x)                                                .′ (x)
                                                                            f




                           .       x
                                   .                                           .                  x
                                                                                                  .




                                                                    .   .          .   .      .       .

  V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative             September 27, 2010      37 / 46
Differentiability FAIL
Vertical Tangents


Example
Let f have the graph on the left-hand side below. Sketch the graph of
the derivative f′ .
                        f
                        .(x)                                                .′ (x)
                                                                            f




                           .       x
                                   .                                           .                  x
                                                                                                  .




                                                                    .   .          .   .      .       .

  V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative             September 27, 2010      37 / 46
Differentiability FAIL
Weird, Wild, Stuff

Example

                        f
                        .(x)




                           .       x
                                   .




 This function is differentiable
 at 0.
                                                                    .   .     .      .      .    .

  V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   38 / 46
Differentiability FAIL
Weird, Wild, Stuff

Example

                        f
                        .(x)                                                .′ (x)
                                                                            f




                           .       x
                                   .                                           .                  x
                                                                                                  .




 This function is differentiable
 at 0.
                                                                    .   .          .   .      .       .

  V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative             September 27, 2010      38 / 46
Differentiability FAIL
Weird, Wild, Stuff

Example

                        f
                        .(x)                                                .′ (x)
                                                                            f




                           .       x
                                   .                                           .                  x
                                                                                                  .




 This function is differentiable
 at 0.
                                                                    .   .          .   .      .       .

  V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative             September 27, 2010      38 / 46
Differentiability FAIL
Weird, Wild, Stuff

Example

                        f
                        .(x)                                                .′ (x)
                                                                            f




                           .       x
                                   .                                           .                  x
                                                                                                  .




 This function is differentiable                       But the derivative is not
 at 0.                                                 continuous at 0!
                                                                    .   .          .   .      .       .

  V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative             September 27, 2010      38 / 46
Outline

Rates of Change
   Tangent Lines
   Velocity
   Population growth
   Marginal costs

The derivative, defined
  Derivatives of (some) power functions
  What does f tell you about f′ ?

How can a function fail to be differentiable?

Other notations

The second derivative

                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   39 / 46
Notation



      Newtonian notation

                                    f′ (x)         y′ (x)          y′

      Leibnizian notation
                                  dy           d                   df
                                                  f(x)
                                  dx           dx                  dx
These all mean the same thing.




                                                                    .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative            September 27, 2010   40 / 46
Meet the Mathematician: Isaac Newton




       English, 1643–1727
       Professor at Cambridge
       (England)
       Philosophiae Naturalis
       Principia Mathematica
       published 1687




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   41 / 46
Meet the Mathematician: Gottfried Leibniz




       German, 1646–1716
       Eminent philosopher as
       well as mathematician
       Contemporarily disgraced
       by the calculus priority
       dispute




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   42 / 46
Outline

Rates of Change
   Tangent Lines
   Velocity
   Population growth
   Marginal costs

The derivative, defined
  Derivatives of (some) power functions
  What does f tell you about f′ ?

How can a function fail to be differentiable?

Other notations

The second derivative

                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   43 / 46
The second derivative



If f is a function, so is f′ , and we can seek its derivative.

                                         f′′ = (f′ )′

It measures the rate of change of the rate of change!




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   44 / 46
The second derivative



If f is a function, so is f′ , and we can seek its derivative.

                                         f′′ = (f′ )′

It measures the rate of change of the rate of change! Leibnizian
notation:
                        d2 y    d2           d2 f
                                    f(x)
                        dx2    dx2           dx2




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   44 / 46
function, derivative, second derivative

                                       y
                                       .
                                                                            .(x) = x2
                                                                            f




                                                                            .′ (x) = 2x
                                                                            f


                                                                            .′′ (x) = 2
                                                                            f
                                        .                                     x
                                                                              .




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   45 / 46
What have we learned today?




      The derivative measures instantaneous rate of change
      The derivative has many interpretations: slope of the tangent line,
      velocity, marginal quantities, etc.
      The derivative reflects the monotonicity (increasing or decreasing)
      of the graph




                                                                   .   .     .      .      .    .

 V63.0121.041, Calculus I (NYU)   Section 2.1–2.2 The Derivative           September 27, 2010   46 / 46

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Lesson 7: The Derivative (Section 41 slides)

  • 1. Section 2.1–2.2 The Derivative and Rates of Change The Derivative as a Function V63.0121.041, Calculus I New York University September 27, 2010 Announcements Quiz this week in recitation on §§1.1–1.4 Get-to-know-you/photo due Friday October 1 . . . . . .
  • 2. Announcements Quiz this week in recitation on §§1.1–1.4 Get-to-know-you/photo due Friday October 1 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 2 / 46
  • 3. Format of written work Please: Use scratch paper and copy your final work onto fresh paper. Use loose-leaf paper (not torn from a notebook). Write your name, lecture section, assignment number, recitation, and date at the top. Staple your homework together. See the website for more information. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 3 / 46
  • 4. Objectives for Section 2.1 Understand and state the definition of the derivative of a function at a point. Given a function and a point in its domain, decide if the function is differentiable at the point and find the value of the derivative at that point. Understand and give several examples of derivatives modeling rates of change in science. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 4 / 46
  • 5. Objectives for Section 2.2 Given a function f, use the definition of the derivative to find the derivative function f’. Given a function, find its second derivative. Given the graph of a function, sketch the graph of its derivative. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 5 / 46
  • 6. Outline Rates of Change Tangent Lines Velocity Population growth Marginal costs The derivative, defined Derivatives of (some) power functions What does f tell you about f′ ? How can a function fail to be differentiable? Other notations The second derivative . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 6 / 46
  • 7. The tangent problem Problem Given a curve and a point on the curve, find the slope of the line tangent to the curve at that point. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 7 / 46
  • 8. The tangent problem Problem Given a curve and a point on the curve, find the slope of the line tangent to the curve at that point. Example Find the slope of the line tangent to the curve y = x2 at the point (2, 4). . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 7 / 46
  • 9. Graphically and numerically y . x2 − 22 x m= x−2 . . 4 . . . x . 2 . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 10. Graphically and numerically y . x2 − 22 x m= x−2 3 . . 9 . . . 4 . . . . x . 2 . 3 . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 11. Graphically and numerically y . x2 − 22 x m= x−2 3 5 . . 9 . . . 4 . . . . x . 2 . 3 . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 12. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 . .25 . 6 . . . 4 . . . . x . 22 . . .5 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 13. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 . .25 . 6 . . . 4 . . . . x . 22 . . .5 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 14. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 . .41 . 4 . . . 4 . . .. x . 2 .. .1 2 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 15. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 . .41 . 4 . . . 4 . . .. x . 2 .. .1 2 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 16. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 . .0401 . 4 4 . . . . x . 2. . .01 2 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 17. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 4.01 . .0401 . 4 4 . . . . x . 2. . .01 2 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 18. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 4.01 . . 4 . 1 . . 1 . . . . x . 1 . 2 . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 19. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 4.01 . . 4 . 1 3 . . 1 . . . . x . 1 . 2 . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 20. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 4.01 . . 4 . 1.5 . .25 . 2 . 1 3 . . . x . 1 2 . .5 . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 21. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 4.01 . . 4 . 1.5 3.5 . .25 . 2 . 1 3 . . . x . 1 2 . .5 . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 22. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 4.01 . . 4 . . .61 . 3 . 1.9 1.5 3.5 1 3 . .. x . 1. . .9 2 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 23. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 4.01 . . 4 . . .61 . 3 . 1.9 3.9 1.5 3.5 1 3 . .. x . 1. . .9 2 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 24. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 4.01 1.99 . .9601 . 3 4 . . 1.9 3.9 1.5 3.5 1 3 . . x . 1. . .99 2 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 25. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 4.01 1.99 3.99 . .9601 . 3 4 . . 1.9 3.9 1.5 3.5 1 3 . . x . 1. . .99 2 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 26. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 4.01 1.99 3.99 . . 4 . 1.9 3.9 1.5 3.5 1 3 . . x . 2 . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 27. Graphically and numerically y . x2 − 22 x m= x−2 3 5 . . 9 . 2.5 4.5 2.1 4.1 2.01 4.01 . .25 . 6 . limit . .41 . 4 . 1.99 3.99 . .0401 . 4.9601 . 3 . .61 3 4 . .. 1.9 3.9 1.5 3.5 . .25 . 2 . 1 3 . . 1 . . . . ... . . x . 1 1 . .. .1 3 . . .5 .99 .5 . 12 . 2.9 2 2 .01 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 28. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 4.01 4 1.99 3.99 . . 4 . 1.9 3.9 1.5 3.5 1 3 . . x . 2 . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 8 / 46
  • 29. The tangent problem Problem Given a curve and a point on the curve, find the slope of the line tangent to the curve at that point. Example Find the slope of the line tangent to the curve y = x2 at the point (2, 4). Upshot If the curve is given by y = f(x), and the point on the curve is (a, f(a)), then the slope of the tangent line is given by f(x) − f(a) mtangent = lim x→a x−a . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 9 / 46
  • 30. Velocity . Problem Given the position function of a moving object, find the velocity of the object at a certain instant in time. Example Drop a ball off the roof of the Silver Center so that its height can be described by h(t) = 50 − 5t2 where t is seconds after dropping it and h is meters above the ground. How fast is it falling one second after we drop it? . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 10 / 46
  • 31. Numerical evidence h(t) = 50 − 5t2 Fill in the table: h(t) − h(1) t vave = t−1 2 − 15 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 11 / 46
  • 32. Numerical evidence h(t) = 50 − 5t2 Fill in the table: h(t) − h(1) t vave = t−1 2 − 15 1.5 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 11 / 46
  • 33. Numerical evidence h(t) = 50 − 5t2 Fill in the table: h(t) − h(1) t vave = t−1 2 − 15 1.5 − 12.5 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 11 / 46
  • 34. Numerical evidence h(t) = 50 − 5t2 Fill in the table: h(t) − h(1) t vave = t−1 2 − 15 1.5 − 12.5 1.1 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 11 / 46
  • 35. Numerical evidence h(t) = 50 − 5t2 Fill in the table: h(t) − h(1) t vave = t−1 2 − 15 1.5 − 12.5 1.1 − 10.5 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 11 / 46
  • 36. Numerical evidence h(t) = 50 − 5t2 Fill in the table: h(t) − h(1) t vave = t−1 2 − 15 1.5 − 12.5 1.1 − 10.5 1.01 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 11 / 46
  • 37. Numerical evidence h(t) = 50 − 5t2 Fill in the table: h(t) − h(1) t vave = t−1 2 − 15 1.5 − 12.5 1.1 − 10.5 1.01 − 10.05 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 11 / 46
  • 38. Numerical evidence h(t) = 50 − 5t2 Fill in the table: h(t) − h(1) t vave = t−1 2 − 15 1.5 − 12.5 1.1 − 10.5 1.01 − 10.05 1.001 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 11 / 46
  • 39. Numerical evidence h(t) = 50 − 5t2 Fill in the table: h(t) − h(1) t vave = t−1 2 − 15 1.5 − 12.5 1.1 − 10.5 1.01 − 10.05 1.001 − 10.005 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 11 / 46
  • 40. Velocity . Problem Given the position function of a moving object, find the velocity of the object at a certain instant in time. Example Drop a ball off the roof of the Silver Center so that its height can be described by h(t) = 50 − 5t2 where t is seconds after dropping it and h is meters above the ground. How fast is it falling one second after we drop it? Solution The answer is (50 − 5t2 ) − 45 5 − 5t2 5(1 − t)(1 + t) v = lim = lim = lim t→1 t−1 t→1 t − 1 t→1 t−1 = (−5) lim (1 + t) = −5 · 2 = −10 t→1 . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 12 / 46
  • 41. Velocity in general . . = h(t) y Upshot . (t0 ) . h . If the height function is given by h(t), the instantaneous velocity . h ∆ at time t0 is given by . (t0 + ∆t) . h . h(t) − h(t0 ) v = lim t→t0 t − t0 h(t0 + ∆t) − h(t0 ) = lim ∆t→0 ∆t . . . t . ∆ .. t t .0 t . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 13 / 46
  • 42. Population growth Problem Given the population function of a group of organisms, find the rate of growth of the population at a particular instant. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 14 / 46
  • 43. Population growth Problem Given the population function of a group of organisms, find the rate of growth of the population at a particular instant. Example Suppose the population of fish in the East River is given by the function 3et P(t) = 1 + et where t is in years since 2000 and P is in millions of fish. Is the fish population growing fastest in 1990, 2000, or 2010? (Estimate numerically) . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 14 / 46
  • 44. Derivation Solution Let ∆t be an increment in time and ∆P the corresponding change in population: ∆P = P(t + ∆t) − P(t) This depends on ∆t, so ideally we would want ( ) ∆P 1 3et+∆t 3et lim = lim − ∆t→0 ∆t ∆t→0 ∆t 1 + et+∆t 1 + et . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 15 / 46
  • 45. Derivation Solution Let ∆t be an increment in time and ∆P the corresponding change in population: ∆P = P(t + ∆t) − P(t) This depends on ∆t, so ideally we would want ( ) ∆P 1 3et+∆t 3et lim = lim − ∆t→0 ∆t ∆t→0 ∆t 1 + et+∆t 1 + et But rather than compute a complicated limit analytically, let us approximate numerically. We will try a small ∆t, for instance 0.1. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 15 / 46
  • 46. Numerical evidence Solution (Continued) To approximate the population change in year n, use the difference P(t + ∆t) − P(t) quotient , where ∆t = 0.1 and t = n − 2000. ∆t r1990 r2000 r2010 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 16 / 46
  • 47. Numerical evidence Solution (Continued) To approximate the population change in year n, use the difference P(t + ∆t) − P(t) quotient , where ∆t = 0.1 and t = n − 2000. ∆t P(−10 + 0.1) − P(−10) r1990 ≈ 0.1 P(0.1) − P(0) r2000 ≈ 0.1 P(10 + 0.1) − P(10) r2010 ≈ 0.1 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 16 / 46
  • 48. Numerical evidence Solution (Continued) To approximate the population change in year n, use the difference P(t + ∆t) − P(t) quotient , where ∆t = 0.1 and t = n − 2000. ∆t ( ) P(−10 + 0.1) − P(−10) 1 3e−9.9 3e−10 r1990 ≈ = − 0.1 0.1 1 + e−9.9 1 + e−10 ( ) P(0.1) − P(0) 1 3e0.1 3e0 r2000 ≈ = − 0.1 0.1 1 + e0.1 1 + e0 ( ) P(10 + 0.1) − P(10) 1 3e10.1 3e10 r2010 ≈ = − 0.1 0.1 1 + e10.1 1 + e10 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 16 / 46
  • 49. Numerical evidence Solution (Continued) To approximate the population change in year n, use the difference P(t + ∆t) − P(t) quotient , where ∆t = 0.1 and t = n − 2000. ∆t ( ) P(−10 + 0.1) − P(−10) 1 3e−9.9 3e−10 r1990 ≈ = − 0.1 0.1 1 + e−9.9 1 + e−10 = 0.000143229 ( ) P(0.1) − P(0) 1 3e0.1 3e0 r2000 ≈ = − 0.1 0.1 1 + e0.1 1 + e0 ( ) P(10 + 0.1) − P(10) 1 3e10.1 3e10 r2010 ≈ = − 0.1 0.1 1 + e10.1 1 + e10 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 16 / 46
  • 50. Numerical evidence Solution (Continued) To approximate the population change in year n, use the difference P(t + ∆t) − P(t) quotient , where ∆t = 0.1 and t = n − 2000. ∆t ( ) P(−10 + 0.1) − P(−10) 1 3e−9.9 3e−10 r1990 ≈ = − 0.1 0.1 1 + e−9.9 1 + e−10 = 0.000143229 ( ) P(0.1) − P(0) 1 3e0.1 3e0 r2000 ≈ = − 0.1 0.1 1 + e0.1 1 + e0 = 0.749376 ( ) P(10 + 0.1) − P(10) 1 3e10.1 3e10 r2010 ≈ = − 0.1 0.1 1 + e10.1 1 + e10 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 16 / 46
  • 51. Numerical evidence Solution (Continued) To approximate the population change in year n, use the difference P(t + ∆t) − P(t) quotient , where ∆t = 0.1 and t = n − 2000. ∆t ( ) P(−10 + 0.1) − P(−10) 1 3e−9.9 3e−10 r1990 ≈ = − 0.1 0.1 1 + e−9.9 1 + e−10 = 0.000143229 ( ) P(0.1) − P(0) 1 3e0.1 3e0 r2000 ≈ = − 0.1 0.1 1 + e0.1 1 + e0 = 0.749376 ( ) P(10 + 0.1) − P(10) 1 3e10.1 3e10 r2010 ≈ = − 0.1 0.1 1 + e10.1 1 + e10 = 0.0001296 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 16 / 46
  • 52. Population growth . Problem Given the population function of a group of organisms, find the rate of growth of the population at a particular instant. Example Suppose the population of fish in the East River is given by the function 3et P(t) = 1 + et where t is in years since 2000 and P is in millions of fish. Is the fish population growing fastest in 1990, 2000, or 2010? (Estimate numerically) Answer We estimate the rates of growth to be 0.000143229, 0.749376, and 0.0001296. So the population is growing fastest in 2000. . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 17 / 46
  • 53. Population growth in general Upshot The instantaneous population growth is given by P(t + ∆t) − P(t) lim ∆t→0 ∆t . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 18 / 46
  • 54. Marginal costs Problem Given the production cost of a good, find the marginal cost of production after having produced a certain quantity. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 19 / 46
  • 55. Marginal costs Problem Given the production cost of a good, find the marginal cost of production after having produced a certain quantity. Example Suppose the cost of producing q tons of rice on our paddy in a year is C(q) = q3 − 12q2 + 60q We are currently producing 5 tons a year. Should we change that? . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 19 / 46
  • 56. Comparisons Solution C(q) = q3 − 12q2 + 60q Fill in the table: q C(q) 4 5 6 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 20 / 46
  • 57. Comparisons Solution C(q) = q3 − 12q2 + 60q Fill in the table: q C(q) 4 112 5 6 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 20 / 46
  • 58. Comparisons Solution C(q) = q3 − 12q2 + 60q Fill in the table: q C(q) 4 112 5 125 6 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 20 / 46
  • 59. Comparisons Solution C(q) = q3 − 12q2 + 60q Fill in the table: q C(q) 4 112 5 125 6 144 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 20 / 46
  • 60. Comparisons Solution C(q) = q3 − 12q2 + 60q Fill in the table: q C(q) AC(q) = C(q)/q 4 112 5 125 6 144 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 20 / 46
  • 61. Comparisons Solution C(q) = q3 − 12q2 + 60q Fill in the table: q C(q) AC(q) = C(q)/q 4 112 28 5 125 6 144 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 20 / 46
  • 62. Comparisons Solution C(q) = q3 − 12q2 + 60q Fill in the table: q C(q) AC(q) = C(q)/q 4 112 28 5 125 25 6 144 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 20 / 46
  • 63. Comparisons Solution C(q) = q3 − 12q2 + 60q Fill in the table: q C(q) AC(q) = C(q)/q 4 112 28 5 125 25 6 144 24 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 20 / 46
  • 64. Comparisons Solution C(q) = q3 − 12q2 + 60q Fill in the table: q C(q) AC(q) = C(q)/q ∆C = C(q + 1) − C(q) 4 112 28 5 125 25 6 144 24 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 20 / 46
  • 65. Comparisons Solution C(q) = q3 − 12q2 + 60q Fill in the table: q C(q) AC(q) = C(q)/q ∆C = C(q + 1) − C(q) 4 112 28 13 5 125 25 6 144 24 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 20 / 46
  • 66. Comparisons Solution C(q) = q3 − 12q2 + 60q Fill in the table: q C(q) AC(q) = C(q)/q ∆C = C(q + 1) − C(q) 4 112 28 13 5 125 25 19 6 144 24 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 20 / 46
  • 67. Comparisons Solution C(q) = q3 − 12q2 + 60q Fill in the table: q C(q) AC(q) = C(q)/q ∆C = C(q + 1) − C(q) 4 112 28 13 5 125 25 19 6 144 24 31 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 20 / 46
  • 68. Marginal costs Problem Given the production cost of a good, find the marginal cost of production after having produced a certain quantity. Example Suppose the cost of producing q tons of rice on our paddy in a year is C(q) = q3 − 12q2 + 60q We are currently producing 5 tons a year. Should we change that? Answer If q = 5, then C = 125, ∆C = 19, while AC = 25. So we should produce more to lower average costs. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 21 / 46
  • 69. Marginal Cost in General Upshot The incremental cost ∆C = C(q + 1) − C(q) is useful, but is still only an average rate of change. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 22 / 46
  • 70. Marginal Cost in General Upshot The incremental cost ∆C = C(q + 1) − C(q) is useful, but is still only an average rate of change. The marginal cost after producing q given by C(q + ∆q) − C(q) MC = lim ∆q→0 ∆q is more useful since it’s an instantaneous rate of change. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 22 / 46
  • 71. Outline Rates of Change Tangent Lines Velocity Population growth Marginal costs The derivative, defined Derivatives of (some) power functions What does f tell you about f′ ? How can a function fail to be differentiable? Other notations The second derivative . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 23 / 46
  • 72. The definition All of these rates of change are found the same way! . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 24 / 46
  • 73. The definition All of these rates of change are found the same way! Definition Let f be a function and a a point in the domain of f. If the limit f(a + h) − f(a) f(x) − f(a) f′ (a) = lim = lim h→0 h x→a x−a exists, the function is said to be differentiable at a and f′ (a) is the derivative of f at a. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 24 / 46
  • 74. Derivative of the squaring function Example Suppose f(x) = x2 . Use the definition of derivative to find f′ (a). . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 25 / 46
  • 75. Derivative of the squaring function Example Suppose f(x) = x2 . Use the definition of derivative to find f′ (a). Solution f(a + h) − f(a) (a + h)2 − a2 f′ (a) = lim = lim h→0 h h→0 h (a2 + 2ah + h2 ) − a2 2ah + h2 = lim = lim h→0 h h→0 h = lim (2a + h) = 2a. h→0 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 25 / 46
  • 76. Derivative of the reciprocal function Example 1 Suppose f(x) = . Use the x definition of the derivative to find f′ (2). . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 26 / 46
  • 77. Derivative of the reciprocal function Example 1 Suppose f(x) = . Use the x definition of the derivative to find f′ (2). x . Solution 1/x − 1/2 f′ (2) = lim . x→2 x−2 . x . 2−x = lim x→2 2x(x − 2) −1 1 = lim =− x→2 2x 4 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 26 / 46
  • 78. The Sure-Fire Sally Rule (SFSR) for adding Fractions In anticipation of the question, “How did you get that?" a c ad ± bc ± = b d bd So 1 1 2−x − x 2 = 2x x−2 x−2 2−x = 2x(x − 2) . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 27 / 46
  • 79. The Sure-Fire Sally Rule (SFSR) for adding Fractions In anticipation of the question, “How did you get that?" a c ad ± bc ± = b d bd So 1 1 2−x − x 2 = 2x x−2 x−2 2−x = 2x(x − 2) . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 27 / 46
  • 80. What does f tell you about f′ ? If f is a function, we can compute the derivative f′ (x) at each point x where f is differentiable, and come up with another function, the derivative function. What can we say about this function f′ ? . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 28 / 46
  • 81. What does f tell you about f′ ? If f is a function, we can compute the derivative f′ (x) at each point x where f is differentiable, and come up with another function, the derivative function. What can we say about this function f′ ? If f is decreasing on an interval, f′ is negative (technically, nonpositive) on that interval . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 28 / 46
  • 82. Derivative of the reciprocal function Example 1 Suppose f(x) = . Use the x definition of the derivative to find f′ (2). x . Solution 1/x − 1/2 f′ (2) = lim . x→2 x−2 . x . 2−x = lim x→2 2x(x − 2) −1 1 = lim =− x→2 2x 4 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 29 / 46
  • 83. What does f tell you about f′ ? If f is a function, we can compute the derivative f′ (x) at each point x where f is differentiable, and come up with another function, the derivative function. What can we say about this function f′ ? If f is decreasing on an interval, f′ is negative (technically, nonpositive) on that interval If f is increasing on an interval, f′ is positive (technically, nonnegative) on that interval . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 30 / 46
  • 84. Graphically and numerically y . x2 − 22 x m= x−2 3 5 2.5 4.5 2.1 4.1 2.01 4.01 . . 4 . 1 . . 1 . . . . x . 1 . 2 . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 31 / 46
  • 85. What does f tell you about f′ ? . Fact If f is decreasing on (a, b), then f′ ≤ 0 on (a, b). . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 32 / 46
  • 86. What does f tell you about f′ ? . Fact If f is decreasing on (a, b), then f′ ≤ 0 on (a, b). Proof. If f is decreasing on (a, b), and ∆x > 0, then f(x + ∆x) − f(x) f(x + ∆x) < f(x) =⇒ <0 ∆x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 32 / 46
  • 87. What does f tell you about f′ ? . Fact If f is decreasing on (a, b), then f′ ≤ 0 on (a, b). Proof. If f is decreasing on (a, b), and ∆x > 0, then f(x + ∆x) − f(x) f(x + ∆x) < f(x) =⇒ <0 ∆x But if ∆x < 0, then x + ∆x < x, and f(x + ∆x) − f(x) f(x + ∆x) > f(x) =⇒ <0 ∆x still! . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 32 / 46
  • 88. What does f tell you about f′ ? . Fact If f is decreasing on (a, b), then f′ ≤ 0 on (a, b). Proof. If f is decreasing on (a, b), and ∆x > 0, then f(x + ∆x) − f(x) f(x + ∆x) < f(x) =⇒ <0 ∆x But if ∆x < 0, then x + ∆x < x, and f(x + ∆x) − f(x) f(x + ∆x) > f(x) =⇒ <0 ∆x f(x + ∆x) − f(x) still! Either way, < 0, so ∆x f(x + ∆x) − f(x) f′ (x) = lim ≤0 ∆x→0 ∆x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 32 / 46
  • 89. Outline Rates of Change Tangent Lines Velocity Population growth Marginal costs The derivative, defined Derivatives of (some) power functions What does f tell you about f′ ? How can a function fail to be differentiable? Other notations The second derivative . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 33 / 46
  • 90. Differentiability is super-continuity Theorem If f is differentiable at a, then f is continuous at a. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 34 / 46
  • 91. Differentiability is super-continuity Theorem If f is differentiable at a, then f is continuous at a. Proof. We have f(x) − f(a) lim (f(x) − f(a)) = lim · (x − a) x→a x→a x−a f(x) − f(a) = lim · lim (x − a) x→a x−a x→a ′ = f (a) · 0 = 0 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 34 / 46
  • 92. Differentiability is super-continuity Theorem If f is differentiable at a, then f is continuous at a. Proof. We have f(x) − f(a) lim (f(x) − f(a)) = lim · (x − a) x→a x→a x−a f(x) − f(a) = lim · lim (x − a) x→a x−a x→a ′ = f (a) · 0 = 0 Note the proper use of the limit law: if the factors each have a limit at a, the limit of the product is the product of the limits. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 34 / 46
  • 93. Differentiability FAIL Kinks Example Let f have the graph on the left-hand side below. Sketch the graph of the derivative f′ . f .(x) . x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 35 / 46
  • 94. Differentiability FAIL Kinks Example Let f have the graph on the left-hand side below. Sketch the graph of the derivative f′ . f .(x) .′ (x) f . x . . x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 35 / 46
  • 95. Differentiability FAIL Kinks Example Let f have the graph on the left-hand side below. Sketch the graph of the derivative f′ . f .(x) .′ (x) f . . x . . x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 35 / 46
  • 96. Differentiability FAIL Cusps Example Let f have the graph on the left-hand side below. Sketch the graph of the derivative f′ . f .(x) . x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 36 / 46
  • 97. Differentiability FAIL Cusps Example Let f have the graph on the left-hand side below. Sketch the graph of the derivative f′ . f .(x) .′ (x) f . x . . x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 36 / 46
  • 98. Differentiability FAIL Cusps Example Let f have the graph on the left-hand side below. Sketch the graph of the derivative f′ . f .(x) .′ (x) f . x . . x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 36 / 46
  • 99. Differentiability FAIL Cusps Example Let f have the graph on the left-hand side below. Sketch the graph of the derivative f′ . f .(x) .′ (x) f . x . . x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 36 / 46
  • 100. Differentiability FAIL Vertical Tangents Example Let f have the graph on the left-hand side below. Sketch the graph of the derivative f′ . f .(x) . x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 37 / 46
  • 101. Differentiability FAIL Vertical Tangents Example Let f have the graph on the left-hand side below. Sketch the graph of the derivative f′ . f .(x) .′ (x) f . x . . x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 37 / 46
  • 102. Differentiability FAIL Vertical Tangents Example Let f have the graph on the left-hand side below. Sketch the graph of the derivative f′ . f .(x) .′ (x) f . x . . x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 37 / 46
  • 103. Differentiability FAIL Vertical Tangents Example Let f have the graph on the left-hand side below. Sketch the graph of the derivative f′ . f .(x) .′ (x) f . x . . x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 37 / 46
  • 104. Differentiability FAIL Weird, Wild, Stuff Example f .(x) . x . This function is differentiable at 0. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 38 / 46
  • 105. Differentiability FAIL Weird, Wild, Stuff Example f .(x) .′ (x) f . x . . x . This function is differentiable at 0. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 38 / 46
  • 106. Differentiability FAIL Weird, Wild, Stuff Example f .(x) .′ (x) f . x . . x . This function is differentiable at 0. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 38 / 46
  • 107. Differentiability FAIL Weird, Wild, Stuff Example f .(x) .′ (x) f . x . . x . This function is differentiable But the derivative is not at 0. continuous at 0! . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 38 / 46
  • 108. Outline Rates of Change Tangent Lines Velocity Population growth Marginal costs The derivative, defined Derivatives of (some) power functions What does f tell you about f′ ? How can a function fail to be differentiable? Other notations The second derivative . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 39 / 46
  • 109. Notation Newtonian notation f′ (x) y′ (x) y′ Leibnizian notation dy d df f(x) dx dx dx These all mean the same thing. . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 40 / 46
  • 110. Meet the Mathematician: Isaac Newton English, 1643–1727 Professor at Cambridge (England) Philosophiae Naturalis Principia Mathematica published 1687 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 41 / 46
  • 111. Meet the Mathematician: Gottfried Leibniz German, 1646–1716 Eminent philosopher as well as mathematician Contemporarily disgraced by the calculus priority dispute . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 42 / 46
  • 112. Outline Rates of Change Tangent Lines Velocity Population growth Marginal costs The derivative, defined Derivatives of (some) power functions What does f tell you about f′ ? How can a function fail to be differentiable? Other notations The second derivative . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 43 / 46
  • 113. The second derivative If f is a function, so is f′ , and we can seek its derivative. f′′ = (f′ )′ It measures the rate of change of the rate of change! . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 44 / 46
  • 114. The second derivative If f is a function, so is f′ , and we can seek its derivative. f′′ = (f′ )′ It measures the rate of change of the rate of change! Leibnizian notation: d2 y d2 d2 f f(x) dx2 dx2 dx2 . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 44 / 46
  • 115. function, derivative, second derivative y . .(x) = x2 f .′ (x) = 2x f .′′ (x) = 2 f . x . . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 45 / 46
  • 116. What have we learned today? The derivative measures instantaneous rate of change The derivative has many interpretations: slope of the tangent line, velocity, marginal quantities, etc. The derivative reflects the monotonicity (increasing or decreasing) of the graph . . . . . . V63.0121.041, Calculus I (NYU) Section 2.1–2.2 The Derivative September 27, 2010 46 / 46