$\left\{\begin{matrix}a=\frac{\sqrt{\frac{y}{1-x}}}{x}\text{; }a=-\frac{\sqrt{\frac{y}{1-x}}}{x}\text{, }&\left(y\geq 0\text{ and }x<1\text{ and }x\neq 0\right)\text{ or }\left(x>1\text{ and }y\leq 0\right)\\a\in \mathrm{R}\text{, }&\left(x=1\text{ or }x=0\right)\text{ and }y=0\end{matrix}\right.$