Trigonometric equation with quadrangle I need some help solving this. Any hints/ideas? We got a quadrangle, inscribed in a circle. The two opposite corners are $x$ and $y$. (From this we know $x+y=180^{\circ }$)? Find $\cos(x)$ if $\tan(y)=-2$.

**Hint**

$$\tan y=\tan(\pi-x)=-\tan x=-2 \rightarrow \tan x=2$$

Can you finish?