
The circumference of a circle is the perimeter of the circle. The circumference of a circle is the distance around the circle.
The circumference of a circle = πD
Where D is the diameter of the circle.
If r is the radius of the circle and D = 2r then
C = π2r
C = 2πr
Circumference which is also the perimeter of a circle
= \( \scriptsize \pi d \: or \: 2 \pi r \)
Note: the value of x = \( \scriptsize 3.142 \\ \scriptsize or \: \normalsize \frac{22}{7} \\ \scriptsize \: or \: 3\frac{1}{7} \)
Example 1:
Calculate the perimeter of a circle of radius 7cm. Use the value of \( \frac{22}{7}\scriptsize \: for \: \pi \)
Solution:
Perimeter = Circumference = \( \scriptsize 2 \pi r \)
= \( \scriptsize 2 \: \times \: \normalsize \frac{22}{7}\scriptsize \: \times \: 7 \)
= \( \scriptsize 2 \: \times \: \normalsize \frac{22}{\not{7}}\scriptsize \: \times \: \not{7} \)
= \( \scriptsize 2 \: \times \: 22 \)
= \( \scriptsize 44cm \)
Example 2:
Calculate the perimeter of a circle if its:
(a) Diameter is 42m
(b) Radius is 35cm
\( \scriptsize \left( \scriptsize \pi = \normalsize \frac{22}{7} \right) \)
Solution:
(a) Diameter is 42m
Perimeter = πD
= \( \frac{22}{7} \scriptsize \: \times \: 42 \)
= \(\scriptsize 22 \: \times \: 6 \)
= \(\scriptsize 132cm \)
(b) Radius is 35cm
Perimeter = 2πr
= \( \scriptsize 2 \: \times \: \normalsize \frac{22}{7} \scriptsize\: \times \: 35 \)
= \( \scriptsize 2 \: \times \: 22 \: \times \: 5 \)
= \( \scriptsize 44 \: \times \: 5 \)
= \( \scriptsize 120 \:cm \)
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