The Solution:
GIven:
[tex]A=\pi r^2[/tex]Required:
[tex]Find\text{ }r\text{ when }A=81\pi in^2[/tex]Substitute:
[tex]\begin{gathered} \pi r^2=81\pi \\ \\ r^2=\frac{81\pi}{\pi}=81 \end{gathered}[/tex]Take the square root of both sides:
[tex]\begin{gathered} r=\sqrt{81}=\pm9 \\ \\ r=9in\text{ \lparen discard r=-9 since }r>0) \end{gathered}[/tex]Answer:
9 inches
