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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 $\tan(y)=-2$.

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Hint

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

Can you finish?

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    Sure, solved it! Thank you much. It's interesting I did the same thing you wrote but couldn't think how to get further in that moment. :) – 2017-01-04
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    You are very welcome. Don't worry, that happens some times! – 2017-01-04