Dolly

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visits member for 1 year, 10 months
seen Oct 13 '12 at 12:28

Oct
13
awarded  Student
Oct
13
asked Let $m \in \mathbb Z, m>1$, then $\cos(2 \pi/m) \in \mathbb Q$ if and only if $m \in \{1,2,3,4,6\}$.