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reference for finit sum of cotangents
Hi I found a finite summation of cotangents as
$$\sum_{k=0}^{n-1}\cot^2(z + \frac{k\pi}{n}) = n^2 - n + n^2 \cot^2(n z), \quad n > 0$$
in the URL bellow
http://functions.wolfram.com/...
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Reference for a tangent squared sum identity
Can anyone help me find a formal reference for the following identity about the summation of squared tangent function:
$$
\sum_{k=1}^m\tan^2\frac{k\pi}{2m+1} = 2m^2+m,\quad m\in\mathbb{N}^+.
$$
I ...
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