We use pi to calculate the measurements of circles. In these formulas, circumference is represented as C, diameter is represented as D, and radius is represented as r. To find the circumference of a circle, use one of these formulae: C= 2

πr and C =

πD. These two formulae are equal since 2r = D. To find the area of a circle, use the formula: A =

πr
2
​2
​​ When solving problems with pi, make sure to read carefully. The problem might ask you to give the answer in numbers, which means calculate using an approximation, like 3.14. Or the problem might ask you to give your answer in terms of pi, which means leaving pi as a symbol.

Example: 4, 1, 5, 1, 6, 5, 3, 6, 5

4 + 1 + 5 + 1 + 6 + 5 + 3 + 6 + 5 = 36

36 ÷ 9 = 4

What is the area of a circle with a radius of 4 cm? Give your answer in terms of pi.

A
8

π cm

B
50.24 cm
2
​2
​​

C
25.12 cm
2
​2
​​

D
16

π cm
2
​2
​​