The volume of a cylinder = cross-section area x length of the cylinder.
Let L= length of the cylinder.
So, volume of cylinder, is: V = (Pi x r^2) x L
Given the volume of the cylinder and rearranging the previous line’s formula, we obtain the formula: r^2 = V/( Pi x L)
And finally, taking the square root of each side of the last formula, we obtain the formula: r = square root of [V/(Pi x L)]