$$\left. \begin{array} { l } { 4 } \\ { \frac { x } { y } + \frac { y } { x } = - 1 ( x , y \neq 0 ) , x ^ { 3 } - y ^ { 3 } } \end{array} \right.$$
$x=\frac{-\sqrt{-3y^{2}}-y}{2}\text{, }y\neq 0\text{, }z=4$
$x=\frac{\sqrt{-3y^{2}}-y}{2}\text{, }y\neq 0\text{, }z=4$
