Answer
$$r=-\sqrt{\frac{A}{A_0}}+1,\:r=\sqrt{\frac{A}{A_0}}+1$$
Work Step by Step
Solving the equation by isolating the given variable using the rules of Algebra, we find:
$$ \left(1-r\right)^2=\frac{A}{A_0}\\ r=-\sqrt{\frac{A}{A_0}}+1,\:r=\sqrt{\frac{A}{A_0}}+1$$