$$

|x|=\begin{cases}x,&\quad x \geq 0 \\-x，&\quad x <0\end{cases}

$$

$$ f(x)=\begin{cases}x,&x>0\\0,&x=0\\-x,&x<0\end{cases} $$

$$ \left\{ \begin{array}{c} x+y=z\\ x-y=2z\\ x+2y=-z \end{array}\right. $$