$\left\{\begin{matrix}x=\frac{3S_{1}}{5x_{12}}\text{, }&x_{12}\neq 0\\x\in \mathrm{C}\text{, }&S_{1}=0\text{ and }x_{12}=0\end{matrix}\right.$

$S_{1}=\frac{5xx_{12}}{3}$

$\left\{\begin{matrix}x=\frac{3S_{1}}{5x_{12}}\text{, }&x_{12}\neq 0\\x\in \mathrm{R}\text{, }&S_{1}=0\text{ and }x_{12}=0\end{matrix}\right.$