Question

You unfold chicken wire to make a circular pen. The area of the pen is 78.5 square meters. How many meters of chicken wire do you need to make the pen?

Answer 1

Circular pen means you are going to use Area = Pi(r^2). 

So 78.5 = pi(r^2). Divide by pi on both sides to get r^2 = 78.5/pi. Take the square root of that to get r or radius = approximately 4.99. If you need an exact answer then use $\sqrt{ \frac{78.5}{ \pi } }$ for the next part because you are not done yet.

Plug whatever R you use into the circumference formula of Circumference = 2(pi)(r).

If you use 4.99, you do 2(pi)(4.99) which is approximately 31.35. If you use the exact answer, you do 2(pi)($\sqrt{ \frac{78.5}{ \pi} }$) which gives you approximately 31.41. 

Either way you'll need about 31 meters of chicken wire but the 31.40796 or ~31.41 is closer to the exact answer.