trigonometry - Trigonometric identity of finite terms

Prove that:

$$\dfrac{1}{\cos x+\cos {3x}} + \dfrac{1}{\cos x+ \cos {5x}}+\dots+\dfrac{1}{\cos x+ \cos {(2n+1)x}} \\= \frac{1}{2}\csc x \,[ \tan{(n+1)x}-\tan{x}]$$

I tried to prove this using the regular formulas. But failed. Please help me.