Minkowski space/Observer vector/Same direction/Exercise

Let ${\displaystyle {}V}$ be a Minkowski space with the Minkowski form ${\displaystyle {}\left\langle -,-\right\rangle }$, and let ${\displaystyle {}v,w}$ be observer vectors from the same half cone. Show

${\displaystyle {}\left\langle v,w\right\rangle <0\,.}$