The formula of the area of the circle:
[tex]A_O=\pi r^2[/tex]
The formula of the circumference of the circle:
[tex]C_O=2\pi r[/tex]
We have:
[tex]A_O=81\pi\ ft.^2[/tex]
therefore:
[tex]\pi r^2=81\pi\ \ \ |:\pi\\\\r^2=81\to r=\sqrt{81}\to r=9\ ft.[/tex]
substitute:
[tex]C_O=2\pi\cdot9=18\pi\ ft.^2[/tex]
