Démonstration élémentaire de la formule $\frac{\pi^2}{\sin^2 x\pi} = \sum_{\nu= -\infty}{\infty}\frac{1}{(x+\nu)^2}$(French) [Elementary proof of the formula $\frac{\pi^2}{\sin^2 x\pi} = \sum_{\nu= -\infty}{\infty}\frac{1}{(x+\nu)^2}$].Enseign. Math. 5(1903), 450--453