$$c = \frac { V _ { a } - V _ { r } } { V _ { r } }$$
$V_{a}=V_{r}\left(c+1\right)$
$V_{r}\neq 0$
$\left\{\begin{matrix}V_{r}=\frac{V_{a}}{c+1}\text{, }&V_{a}\neq 0\text{ and }c\neq -1\\V_{r}\neq 0\text{, }&V_{a}=0\text{ and }c=-1\end{matrix}\right.$
