Slides for Functions lecture 1. JC H2 Maths A levels Singapore www.mathacademy.sg Copyright 2015 Math Academy

1. Functions Lesson 1
www.MathAcademy.sg
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2. Set notations
[a, b] = {x ∈ R : a ≤ x ≤ b}
(a, b] = {x ∈ R : a < x ≤ b}
(a, b) = {x ∈ R : a < x < b}
(a, ∞) = {x ∈ R : x > a}
R = set of real numbers
R+
= set of positive real numbers
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3. Rule, domain and range
X f (X)
1
2
3
3
4
5
f
Domain Range
A function f , is defined by its rule and domain.
f : x → x + 2
rule
, x ∈ [0, ∞).
domain
f (x) = x + 2, x ≥ 0.
Remark: When we state a function, we must always state both its rule and
domain.
The domain Df , is the set of all possible x values.
The range Rf , is the set of all possible y values.c⃝ 2015 Math Academy www.MathAcademy.sg 3

4. Rule, domain and range
X f (X)
1
2
3
3
4
5
f
Domain Range
A function f , is defined by its rule and domain.
f : x → x + 2
rule
, x ∈ [0, ∞).
domain
f (x) = x + 2, x ≥ 0.
Remark: When we state a function, we must always state both its rule and
domain.
The domain Df , is the set of all possible x values.
The range Rf , is the set of all possible y values.c⃝ 2015 Math Academy www.MathAcademy.sg 4

5. Finding Range
Sketch the graph to find Rf . Rf is the range of y-values that the graph takes.
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6. Example (1)
Find the range of the following:
(a) f : x → x2
− 1, x ∈ [−1, 2], (b) g : x → ex
+ 1, x ∈ R.
Solution:
(a)
−1 1 2
−1
3
x
y
∴ Rf = [−1, 3].
(b)
1
x
y
∴ Rg = (1, ∞).
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7. Horizontal line test
A function is said to be 1 − 1 if for every y ∈ Rf , there is only ONE x such
that f (x) = y.
−1 1
1
x
y
It is not 1-1 since the line y = 1
cuts the graph twice.
x
y
It is 1-1 since every horizontal line
cuts the graph at most once.
Horizontal Line Test
f is NOT a 1 − 1 function if there is a horizontal line that cuts the graph at
MORE THAN ONE point.
f IS a 1 − 1 function if any horizontal line y = a, where a ∈ Rf , cuts the graph
AT ONLY ONE point.
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8. Horizontal line test
A function is said to be 1 − 1 if for every y ∈ Rf , there is only ONE x such
that f (x) = y.
−1 1
1
x
y
It is not 1-1 since the line y = 1
cuts the graph twice.
x
y
It is 1-1 since every horizontal line
cuts the graph at most once.
Horizontal Line Test
f is NOT a 1 − 1 function if there is a horizontal line that cuts the graph at
MORE THAN ONE point.
f IS a 1 − 1 function if any horizontal line y = a, where a ∈ Rf , cuts the graph
AT ONLY ONE point.
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9. Differentiation test for 1-1
f is 1 − 1 if
f ′
(x) > 0 for ALL x in the domain (strictly increasing functions)
or
f ′
(x) < 0 for ALL x in the domain (strictly decreasing functions)
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10. Example (2)
The function f is defined by
f : x → | − x2
− 2x + 3|, x ∈ R.
(a) With the aid of a diagram, explain why f is not 1-1. [2]
Show not 1-1 through graph
Sketch the graph, give the equation of a SPECIFIC horizontal line that cuts
the graph in at least 2 points.
(a)
−3 1
3
y = f (x)
x
y
The line y = 1 cuts the graph of y = f (x) more than once, therefore f is
not 1-1.
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11. Example (2)
The function f is defined by
f : x → | − x2
− 2x + 3|, x ∈ R.
(a) With the aid of a diagram, explain why f is not 1-1. [2]
Show not 1-1 through graph
Sketch the graph, give the equation of a SPECIFIC horizontal line that cuts
the graph in at least 2 points.
(a)
−3 1
3
y = f (x)
x
y
The line y = 1 cuts the graph of y = f (x) more than once, therefore f is
not 1-1.
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12. Example (2)
The function f is defined by f : x → | − x2
− 2x + 3|, x ∈ R.
(b) If the domain of f is restricted to the set {x ∈ R : x ≥ k}, state with a
reason the least value of k for which the function is 1-1. [2]
(b)
−3 1
3
y = f (x)
x
y
Show 1-1 through graph
Sketch the graph, explain that any horizontal line cuts the graph at only 1
point.
Least value of k is 1. When x ≥ 1, any horizontal line y = a for a ∈ Rf ,
cuts the graph of y = f (x) at only 1 point, hence it is 1-1.
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13. Example (2)
The function f is defined by f : x → | − x2
− 2x + 3|, x ∈ R.
(b) If the domain of f is restricted to the set {x ∈ R : x ≥ k}, state with a
reason the least value of k for which the function is 1-1. [2]
(b)
−3 1
3
y = f (x)
x
y
Show 1-1 through graph
Sketch the graph, explain that any horizontal line cuts the graph at only 1
point.
Least value of k is 1. When x ≥ 1, any horizontal line y = a for a ∈ Rf ,
cuts the graph of y = f (x) at only 1 point, hence it is 1-1.
c⃝ 2015 Math Academy www.MathAcademy.sg 13

14. Example (2)
The function f is defined by f : x → | − x2
− 2x + 3|, x ∈ R.
(b) If the domain of f is restricted to the set {x ∈ R : x ≥ k}, state with a
reason the least value of k for which the function is 1-1. [2]
(b)
−3 1
3
y = f (x)
x
y
Show 1-1 through graph
Sketch the graph, explain that any horizontal line cuts the graph at only 1
point.
Least value of k is 1. When x ≥ 1, any horizontal line y = a for a ∈ Rf ,
cuts the graph of y = f (x) at only 1 point, hence it is 1-1.
c⃝ 2015 Math Academy www.MathAcademy.sg 14

15. Example (2)
The function f is defined by
f : x → | − x2
− 2x + 3|, x ∈ R.
(c) By considering the derivative of f (x), prove that f is a one-one function
for the domain you have found in (b). [2]
Show 1-1 through differentiation
Show that f ′
(x) is either > 0 or < 0 for ∀x ∈ Df .
block: This method cannot be used to show that a function is not 1-1.
(c) For x ≥ 1, f (x) = x2
+ 2x − 3.
f ′
(x) = 2x + 2
≥ 2(1) + 2 since x ≥ 1
= 4 > 0
Since f ′
(x) > 0 for x ≥ 1, f is a strictly increasing function, and hence it is
1-1.
c⃝ 2015 Math Academy www.MathAcademy.sg 15

16. Example (2)
The function f is defined by
f : x → | − x2
− 2x + 3|, x ∈ R.
(c) By considering the derivative of f (x), prove that f is a one-one function
for the domain you have found in (b). [2]
Show 1-1 through differentiation
Show that f ′
(x) is either > 0 or < 0 for ∀x ∈ Df .
block: This method cannot be used to show that a function is not 1-1.
(c) For x ≥ 1, f (x) = x2
+ 2x − 3.
f ′
(x) = 2x + 2
≥ 2(1) + 2 since x ≥ 1
= 4 > 0
Since f ′
(x) > 0 for x ≥ 1, f is a strictly increasing function, and hence it is
1-1.
c⃝ 2015 Math Academy www.MathAcademy.sg 16

17. Example (2)
The function f is defined by
f : x → | − x2
− 2x + 3|, x ∈ R.
(c) By considering the derivative of f (x), prove that f is a one-one function
for the domain you have found in (b). [2]
Show 1-1 through differentiation
Show that f ′
(x) is either > 0 or < 0 for ∀x ∈ Df .
block: This method cannot be used to show that a function is not 1-1.
(c) For x ≥ 1, f (x) = x2
+ 2x − 3.
f ′
(x) = 2x + 2
≥ 2(1) + 2 since x ≥ 1
= 4 > 0
Since f ′
(x) > 0 for x ≥ 1, f is a strictly increasing function, and hence it is
1-1.
c⃝ 2015 Math Academy www.MathAcademy.sg 17

18. Example (2)
The function f is defined by
f : x → | − x2
− 2x + 3|, x ∈ R.
(c) By considering the derivative of f (x), prove that f is a one-one function
for the domain you have found in (b). [2]
Show 1-1 through differentiation
Show that f ′
(x) is either > 0 or < 0 for ∀x ∈ Df .
block: This method cannot be used to show that a function is not 1-1.
(c) For x ≥ 1, f (x) = x2
+ 2x − 3.
f ′
(x) = 2x + 2
≥ 2(1) + 2 since x ≥ 1
= 4 > 0
Since f ′
(x) > 0 for x ≥ 1, f is a strictly increasing function, and hence it is
1-1.
c⃝ 2015 Math Academy www.MathAcademy.sg 18

19. Inverse functions
Df Rf
x y
f
f (x) = y
f −1
f −1
(y) = x
Properties of inverse function
1. For f −1
to exist, f must be a 1-1 function.
2. Df −1 = Rf .
3. Rf −1 = Df .
4. (f −1
)−1
= f .
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20. Inverse functions
Geometrical relationship between a function and its inverse
(i) The graph of f −1
is the reflection of the graph f about the line y = x.
(ii) (a, b) lies on f ⇔ (b, a) lies on f −1
.
(iii) x = k is an asymptote of f ⇔ y = k is an asymptote of f −1
Remark: The notation f −1
stands for the inverse function of f . It is not the
same as 1
f
.
c⃝ 2015 Math Academy www.MathAcademy.sg 20

21. Inverse functions
Geometrical relationship between a function and its inverse
(i) The graph of f −1
is the reflection of the graph f about the line y = x.
(ii) (a, b) lies on f ⇔ (b, a) lies on f −1
.
(iii) x = k is an asymptote of f ⇔ y = k is an asymptote of f −1
Remark: The notation f −1
stands for the inverse function of f . It is not the
same as 1
f
.
c⃝ 2015 Math Academy www.MathAcademy.sg 21

22. Inverse functions
Geometrical relationship between a function and its inverse
(i) The graph of f −1
is the reflection of the graph f about the line y = x.
(ii) (a, b) lies on f ⇔ (b, a) lies on f −1
.
(iii) x = k is an asymptote of f ⇔ y = k is an asymptote of f −1
Remark: The notation f −1
stands for the inverse function of f . It is not the
same as 1
f
.
c⃝ 2015 Math Academy www.MathAcademy.sg 22

23. Inverse functions
Geometrical relationship between a function and its inverse
(i) The graph of f −1
is the reflection of the graph f about the line y = x.
(ii) (a, b) lies on f ⇔ (b, a) lies on f −1
.
(iii) x = k is an asymptote of f ⇔ y = k is an asymptote of f −1
Remark: The notation f −1
stands for the inverse function of f . It is not the
same as 1
f
.
c⃝ 2015 Math Academy www.MathAcademy.sg 23

24. Example (3)
The function f is defined by f : x → x2
− 8x + 17 for x > 4.
(i) Sketch the graph of y = f (x). Your sketch should indicate the position of
the graph in relation to the origin.
(i)
4
1
y = f (x)
x
y
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25. Example (3)
The function f is defined by f : x → x2
− 8x + 17 for x > 4.
(i) Sketch the graph of y = f (x). Your sketch should indicate the position of
the graph in relation to the origin.
(i)
4
1
y = f (x)
x
y
c⃝ 2015 Math Academy www.MathAcademy.sg 25

26. Example (3)
The function f is defined by f : x → x2
− 8x + 17 for x > 4.
(ii) Show that the inverse function f −1
exists and find f −1
(x) in similar form.
Showing inverse exists
To show f −1
exists, we only need to show that f is 1-1.
(ii) Every horizontal line y = a for a > 1 cuts the graph of y = f (x) at only 1
point, hence it is 1-1 and f −1
exists.
y = x2
− 8x + 17
= x2
− 8x + (−4)2
− (−4)2
+ 17
= (x − 4)2
+ 1
x − 4 = ±
√
y − 1
x = ±
√
y − 1 + 4
x =
√
y − 1 + 4 or −
√
y − 1 + 4
[rej since x > 4]
∴ f −1
(x) =
√
x − 1 + 4, x > 1.
Note: You must state the domain of f −1
!c⃝ 2015 Math Academy www.MathAcademy.sg 26

27. Example (3)
The function f is defined by f : x → x2
− 8x + 17 for x > 4.
(ii) Show that the inverse function f −1
exists and find f −1
(x) in similar form.
Showing inverse exists
To show f −1
exists, we only need to show that f is 1-1.
(ii) Every horizontal line y = a for a > 1 cuts the graph of y = f (x) at only 1
point, hence it is 1-1 and f −1
exists.
y = x2
− 8x + 17
= x2
− 8x + (−4)2
− (−4)2
+ 17
= (x − 4)2
+ 1
x − 4 = ±
√
y − 1
x = ±
√
y − 1 + 4
x =
√
y − 1 + 4 or −
√
y − 1 + 4
[rej since x > 4]
∴ f −1
(x) =
√
x − 1 + 4, x > 1.
Note: You must state the domain of f −1
!c⃝ 2015 Math Academy www.MathAcademy.sg 27

28. Example (3)
The function f is defined by f : x → x2
− 8x + 17 for x > 4.
(ii) Show that the inverse function f −1
exists and find f −1
(x) in similar form.
Showing inverse exists
To show f −1
exists, we only need to show that f is 1-1.
(ii) Every horizontal line y = a for a > 1 cuts the graph of y = f (x) at only 1
point, hence it is 1-1 and f −1
exists.
y = x2
− 8x + 17
= x2
− 8x + (−4)2
− (−4)2
+ 17
= (x − 4)2
+ 1
x − 4 = ±
√
y − 1
x = ±
√
y − 1 + 4
x =
√
y − 1 + 4 or −
√
y − 1 + 4
[rej since x > 4]
∴ f −1
(x) =
√
x − 1 + 4, x > 1.
Note: You must state the domain of f −1
!c⃝ 2015 Math Academy www.MathAcademy.sg 28

29. Example (3)
The function f is defined by f : x → x2
− 8x + 17 for x > 4.
(ii) Show that the inverse function f −1
exists and find f −1
(x) in similar form.
Showing inverse exists
To show f −1
exists, we only need to show that f is 1-1.
(ii) Every horizontal line y = a for a > 1 cuts the graph of y = f (x) at only 1
point, hence it is 1-1 and f −1
exists.
y = x2
− 8x + 17
= x2
− 8x + (−4)2
− (−4)2
+ 17
= (x − 4)2
+ 1
x − 4 = ±
√
y − 1
x = ±
√
y − 1 + 4
x =
√
y − 1 + 4 or −
√
y − 1 + 4
[rej since x > 4]
∴ f −1
(x) =
√
x − 1 + 4, x > 1.
Note: You must state the domain of f −1
!c⃝ 2015 Math Academy www.MathAcademy.sg 29

30. Example (3)
The function f is defined by f : x → x2
− 8x + 17 for x > 4.
(ii) Show that the inverse function f −1
exists and find f −1
(x) in similar form.
Showing inverse exists
To show f −1
exists, we only need to show that f is 1-1.
(ii) Every horizontal line y = a for a > 1 cuts the graph of y = f (x) at only 1
point, hence it is 1-1 and f −1
exists.
y = x2
− 8x + 17
= x2
− 8x + (−4)2
− (−4)2
+ 17
= (x − 4)2
+ 1
x − 4 = ±
√
y − 1
x = ±
√
y − 1 + 4
x =
√
y − 1 + 4 or −
√
y − 1 + 4
[rej since x > 4]
∴ f −1
(x) =
√
x − 1 + 4, x > 1.
Note: You must state the domain of f −1
!c⃝ 2015 Math Academy www.MathAcademy.sg 30

31. Example (3)
The function f is defined by f : x → x2
− 8x + 17 for x > 4.
(iii) On the same diagram as in part (i), sketch the graph of y = f −1
.
(iii) (i) The graph of f −1 is the reflection of the graph f about the line y = x.
(ii) (a, b) lies on f ⇔ (b, a) lies on f −1.
(iii) x = k is an asymptote of f ⇔ y = k is an asymptote of f −1
1 4
1
4
y = f (x)
y = f −1
(x)
y = x
x
y
c⃝ 2015 Math Academy www.MathAcademy.sg 31

32. Example (3)
The function f is defined by f : x → x2
− 8x + 17 for x > 4.
(iv) Write down the equation of the line in which the graph of y = f (x) must
be reflected in order to obtain the graph of y = f −1
, and hence find the
exact solution of the equation f (x) = f −1
(x).
(iv) It must be reflected along the line y = x. Since f and f −1
intersect at the
line y = x, finding exact solution of the equation f (x) = f −1
(x) is
equivalent to finding
f (x) = x
x2
− 8x + 17 = x
x2
− 9x + 17 = 0
x =
9 ±
√
92 − 4(1)(17)
2
=
9 +
√
13
2
or
9 −
√
13
2
(rej as it is not in Df )
c⃝ 2015 Math Academy www.MathAcademy.sg 32

33. Example (3)
The function f is defined by f : x → x2
− 8x + 17 for x > 4.
(iv) Write down the equation of the line in which the graph of y = f (x) must
be reflected in order to obtain the graph of y = f −1
, and hence find the
exact solution of the equation f (x) = f −1
(x).
(iv) It must be reflected along the line y = x. Since f and f −1
intersect at the
line y = x, finding exact solution of the equation f (x) = f −1
(x) is
equivalent to finding
f (x) = x
x2
− 8x + 17 = x
x2
− 9x + 17 = 0
x =
9 ±
√
92 − 4(1)(17)
2
=
9 +
√
13
2
or
9 −
√
13
2
(rej as it is not in Df )
c⃝ 2015 Math Academy www.MathAcademy.sg 33

34. Example (3)
The function f is defined by f : x → x2
− 8x + 17 for x > 4.
(iv) Write down the equation of the line in which the graph of y = f (x) must
be reflected in order to obtain the graph of y = f −1
, and hence find the
exact solution of the equation f (x) = f −1
(x).
(iv) It must be reflected along the line y = x. Since f and f −1
intersect at the
line y = x, finding exact solution of the equation f (x) = f −1
(x) is
equivalent to finding
f (x) = x
x2
− 8x + 17 = x
x2
− 9x + 17 = 0
x =
9 ±
√
92 − 4(1)(17)
2
=
9 +
√
13
2
or
9 −
√
13
2
(rej as it is not in Df )
c⃝ 2015 Math Academy www.MathAcademy.sg 34

35. Example (4)
Function g is defined by
g : x → x
2
− 3x for x ∈ R,
If the domain of g is restricted to the set {x ∈ R : x ≥ a}, find the least value of a for which g−1
exists. Hence, find g−1
and state its domain. [4]
[a = 1.5, g−1
(x) = 3
2 +
√
x + 9
4 , x ≥ − 9
4 ]
0 3
(1.5, -2.25)
y = g(x)
From the graph, g is 1-1 for x ≥ 1.5. Hence, g−1
exists when a = 1.5
y = x
2
− 3x
= (x − 1.5)
2
− 2.25
y + 2.25 = (x − 1.5)
2
x − 1.5 =
√
y + 2.25 or −
√
y + 2.25
x = 1.5 +
√
y + 2.25 or 1.5 −
√
y + 2.25( rej since x ≥ 1.5)
∴ g−1
x = 1.5 +
√
x + 2.25, x ≥ −2.25c⃝ 2015 Math Academy www.MathAcademy.sg 35

36. Example (4)
Function g is defined by
g : x → x
2
− 3x for x ∈ R,
If the domain of g is restricted to the set {x ∈ R : x ≥ a}, find the least value of a for which g−1
exists. Hence, find g−1
and state its domain. [4]
[a = 1.5, g−1
(x) = 3
2 +
√
x + 9
4 , x ≥ − 9
4 ]
0 3
(1.5, -2.25)
y = g(x)
From the graph, g is 1-1 for x ≥ 1.5. Hence, g−1
exists when a = 1.5
y = x
2
− 3x
= (x − 1.5)
2
− 2.25
y + 2.25 = (x − 1.5)
2
x − 1.5 =
√
y + 2.25 or −
√
y + 2.25
x = 1.5 +
√
y + 2.25 or 1.5 −
√
y + 2.25( rej since x ≥ 1.5)
∴ g−1
x = 1.5 +
√
x + 2.25, x ≥ −2.25c⃝ 2015 Math Academy www.MathAcademy.sg 36