Q: Prove the Identity - Double Angle $\cot(A) =\sin(2A)/(1-\cos(2A))$
I've tried converting the top and bottom by 
every single one of their equivalent formulas but didn't have any luck. 
Thank you

A: $$\begin {align}\frac {\sin(2A)}{1-\cos(2A)}&=\frac{2\sin A \cos A}{1-\cos^2A+\sin^2A}\\
&=\frac {2 \sin A \cos A}{2\sin^2A}\\ &=\cot A \end {align}$$

A: Hint:
You have the linearisation formula:
$$\sin^2A==\frac{1-\cos 2A}2.$$
