Školjka

%V0 Dokažite da za svaka tri realna broja $x$, $y$, $z$ vrijedi nejednakost $$\left\vert x \right\vert + \left\vert y \right\vert + \left\vert z \right\vert - \left\vert x+y \right\vert - \left\vert y+z \right\vert - \left\vert z+x \right\vert + \left\vert x+y+z \right\vert \geqslant 0 \text{.}$$