2. Radius
• The radius of a circle is from the center
to the outer edge of the circle.
• The radius is one half of the diameter
as shown here.
r
3. Diameter
• The diameter goes A
from one edge of
the circle to the
C D
other.
• The diameters is
twice the radius or B
r2. m CD = 2.17 cm
• AB
4. Pi π
• Pi is ≈ 3.14
• Pi is represented by the symbol
π
5. Parts of a circle
A In circle O,
OB is a radius
O AC is a diameter
B
C
6. Parts of a circle
A The distance
around circle O
is called the
O circumference of
B the circle. It is
similar to the
perimeter of a
C polygon.
7. The number π
A
The ratio of the
circumference of
O a circle to its diameter
is the number π.
m CA = 5.8982 cm
C
Circumference OB = 18.5299 cm
(Circumference OB)
= 3.1416
m CA
8. Finding the Circumference
Since we know
d that C/d = π we
can solve for C.
r
C
(d ) = π(d)
d
Therefore: C = π d or
C = 2π r
