Rearrange the formula A = / pi r ^ (2) for r

Consider re-writing this formula as "A = pi*r^2." The backslash symbol "/" is unnecessary.

If A = pi*r^2, and we wish to solve for r, then divide both sides by pi:

A/pi = r^2

Taking the square root of both sides, sqrt (A/pi) = r (r must be positive in this situation).