Question

If the area of a circle is 81pi square feet, find its circumference.

Answer 1

The formula of the area of the circle:


`A_O=\pi r^2`

The formula of the circumference of the circle:


`C_O=2\pi r`

We have:


`A_O=81\pi\ ft.^2`

therefore:


`\pi r^2=81\pi\ \ \ |:\pi\\\\r^2=81\to r=√(81)\to r=9\ ft.`

substitute:


`C_O=2\pi\cdot9=18\pi\ ft.^2`