$\frac{\sin x}{1-\cos x}+\frac{\sin x}{1+\cos x}=\frac{2}{\sin x}$
$(ĐK: x \ne k\pi)$
$⇔\frac{\sin x(1+\cos x)+\sin x(1-\cos x)}{(1-\cos x)(1+\cos x)}=\frac{2}{\sin x}$
$⇔\frac{\sin x+\sin x\cos x+\sin x-\sin x\cos x}{1-\cos x^2}=\frac{2}{\sin x}$
$⇔\frac{2\sin x}{\sin x^2}=\frac{2}{\sin x}$
$⇔\frac{2}{\sin x}=\frac{2}{\sin x}$ (luôn đúng)
