find the perimeter of the circle whose area = 25pi.

First, we have to establish that it is better to say the
"circumference" of the circle, instead of the "perimeter" of the
circle.

We know that the length of the circle, namely it's
circumference, is:

L = 2*pi*r,
where:

L - the length of the circle (
circumference)

pi - 3.14...

r
- the radius of the circle

Wr don't know the area of the
circle, yet.

We could calculate the radius of the circle,
using the formula for the area of the circle:

A =
pi*r^2

25pi = pi*r^2

We'll
divide by pi both sides:

25 =
r^2

sqrt 25 = sqrt r^2

r =
5

Now, we can calculate the
circumference:

L = 2*pi*5

L =
10*pi

L = 10*3.14

The length
is about 314 units.