$$\begin{gathered} \left(12\right) If \frac{2x-3y}{3z+y}=\frac{z-y}{z-x}=\frac{x+3z}{2y-3z} then prove that every ratio =\frac{x}{y} \\ \left(13\right) If \frac{by+cz}{b^2+c^2} = \frac{cz+zx}{c^2+a^2}=\frac{ax+by}{a^2+b^2} then prove that \frac{x}{a}=\frac{y}{b}=\frac{z}{c} \end{gathered}$$