Prove $\sum_{k=1}^m \cot^2 k\pi/(2m+1)=m(2m-1)/3$ 
Prove that $$ \sum_{k=1}^m \cot^2 \frac{k\pi}{2m+1}=\frac{m(2m-1)}{3} $$

I have tried to use $$\sin\left((2m+1)x\right)=
\left(\sin^{2m+1}x\right) \cdot \left(\sum_{j=0}^m (-1)^j \binom{2m+1}{2j+1}\left(\cot^2x\right)^{m-j}\right)$$
and induction without any success. Thanks for any help!

Further to my last comment and given you used
$$\sin\left((2m+1)x\right)=
\left(\sin^{2m+1}x\right) \left(\sum_{j=0}^m (-1)^j \binom{2m+1}{2j+1}(\cot^2x)^{m-j}\right)=
\left(\sin^{2m+1}x\right) \cdot P\left(\cot^2{x}\right)\tag{1}$$
where 
$$P(x)=\sum_{j=0}^m (-1)^j \binom{2m+1}{2j+1}x^{m-j}=\binom{2m+1}{1}x^m-\binom{2m+1}{3}x^{m-1}+...$$
is a polynomial of degree $m$, with (easy to see from $(1)$ since $\sin\left((2m+1)\frac{k\pi}{2m+1}\right)=0$) $\cot^2{\frac{k\pi}{2m+1}}, k=1..m$ as roots. Using Vieta's formulas
$$\sum_{k=1}^m \cot^2{\frac{k\pi}{2m+1}}=-\frac{-\binom{2m+1}{3}}{\binom{2m+1}{1}}=\frac{(2m+1)(2m)(2m-1)}{(2m+1)\cdot 2 \cdot 3}=\frac{m(2m-1)}{3}$$
