The area of a circle (a) is given by the formula a=pi*r2 where r is the circle's radius. the formula to find r is . if and , r is centimeters.

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.
 
 The formula to find r is
 r = root (A / pi)

Where are the functions?