Possible Duplicate:
When is $\sin(x)$ rational?
Let $m \in \mathbb Z, m\geq1$, then $\cos(2 \pi/m) \in \mathbb Q$ if and only if $m \in \{1,2,3,4,6\}$.
Why is this statement true? Why is $\cos(2 \pi/m)$ always non rational for the integer $m >6$?
Thanks very much.