Problems

Does there exist a real number $\alpha$ such that the number $\cos \alpha$ is irrational, and all the numbers $\cos 2\alpha$, $\cos 3\alpha$, $\cos 4\alpha$, $\cos 5\alpha$ are rational?

Prove that for $x\ne \pi n$ ($n$ is an integer) $\sin x$ and $\cos x$ are rational if and only if the number $\tan x/2$ is rational.