How to prove the following trigonometric equation

Question

Possible Duplicate:
Sum of the reciprocal of sine squared

Can anyone help me?

If $\alpha=\frac{\pi}{N}$, where $N\geqslant 2$

How to prove the following equation?

$$\sum_{k=1}^{N-1}\frac1{\sin^2(k\alpha)}=\frac{N^2-1}{3}$$