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:

$\pi r^2=81\pi\ \ \ |:\pi\\\\r^2=81\to r=\sqrt{81}\to r=9\ ft.$

substitute:

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