For this case what you must do is to determine the value of r of the given equation:
A = pi * r ^ 2.
Clearing the value of r we have:
r = root (A / pi)
Note that r has two roots:
r1: = + root (A / pi)
r2: = - root (A / pi)
In this case we must ignore the negative root.
Answer:
The formula to find r is
r = root (A / pi)
