Question

The area of a circle with radius r is given by the formula A=πr2. If a circle has area 81π square inches, what is the radius (in inches)

Answer 1

The Solution:

GIven:

`A=\pi r^2`

Required:

`Find\text{ }r\text{ when }A=81\pi in^2`

Substitute:

`\begin{gathered} \pi r^2=81\pi \\ \\ r^2=(81\pi)/(\pi)=81 \end{gathered}`

Take the square root of both sides:

`\begin{gathered} r=√(81)=\pm9 \\ \\ r=9in\text{ \lparen discard r=-9 since }r>0) \end{gathered}`

Answer:

9 inches