1. ## infinite series

Find the summation of this series :
$\sum_{k=1}^{\infty} \tan^{-1}[ \frac{ y^2 }{ k^2 + x^2 } ]$

Consider $sinx = x \prod_{j=1}^{\infty} [ 1 - \frac{x^2}{(j \pi)^2}]$