Question

Prove that for any integer $\rm{m>1,}$
$\displaystyle}rm\left(z + a\right)^{2m} - \left(z - a\right)^{2m} = 4maz \prod^{m - 1}_{k = 1} \left\{ {z}^{2} + {a}^{2} \: \cot^{2} \left( \frac{k\pi}{2m} \right) \right\}$
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