Hi,

I am going to answer a similar question.

How do you find the radius of a circle if only the perimeter is given to you?

The key here is to know an expression that links the perimeter and radius of a circle. If $P$ is the perimeter of a circle of radius $r$ then

\[P = 2 \pi \; r.\]

Solving this equation for $r$ yields

\[r = \frac{P}{2 \pi}.\]

Thus if you know the perimeter you can calculate the radius.

Now try to see how you can calculate the radius of a circle if you know the area.

Penny