Possible Duplicate:
Identity involving binomial coefficients
how to prove $ \sum_{k=1}^{n}\cot^2\left( \frac{k\pi}{2n+1}\right)=\frac{n(2n-1)}{3} $ another $ \sum_{k=0}^n{\binom {2k} {n} \binom {2n-2k} {n-k}}=4^n $
Possible Duplicate:
Identity involving binomial coefficients
how to prove $ \sum_{k=1}^{n}\cot^2\left( \frac{k\pi}{2n+1}\right)=\frac{n(2n-1)}{3} $ another $ \sum_{k=0}^n{\binom {2k} {n} \binom {2n-2k} {n-k}}=4^n $