# Difficulty proving identity

1. Oct 16, 2006

### rhule009

can anyone help[ me prove the following identity i keep on ending up in a dead end

tan3A=3tanA-tan^3 A/ 1-3tan^2 A
thank you

2. Oct 16, 2006

you know that $$\tan(A+B) = \frac{\tan A + \tan B}{1-\tan A \tan B}$$. Setting $$A = B$$ we get $$\tan 2A = \frac{2\tan A}{1-\tan A \ tan A}$$. So what is $$\tan(A+2A)$$? Thus work from the left to the right.

Last edited: Oct 16, 2006
3. Oct 18, 2006

### rhule009

i end up with
tanA+2tanA/1-tanAtanAtanA

4. Oct 18, 2006

### rhule009

what am i doing wrong