$$\sum_{i=1}^N \frac{1}{\sin^2(\frac{i \pi}{2 N})} = \frac{1}{3}(2N^2+1)$$
How can I prove this ?
Thank you.
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3see Is $\sum_{k=1}^{m-1}\frac{1}{\sin^2\frac{k\pi}{m}}=\frac{m^2-1}{3}$ true for $m\in\mathbb N$? – mathlove Jul 29 '16 at 08:03
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See also : http://math.stackexchange.com/questions/1251060/how-prove-that-frac1-sin2-frac-pi2n-frac1-sin2-frac2-pi2n – lab bhattacharjee Jul 29 '16 at 08:06
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@mathlove Thank you and sorry for my insufficient searching. – Jul 29 '16 at 08:09
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@lab bhattacharjee Thank you and sorry for my insufficient searching. – Jul 29 '16 at 08:10