Start with the formula for $\displaystyle tan(a+ b)= \frac{tan(a)+ tan(b)}{1+ tan(a)tan(b)}$. Use that to find a formula for tan(2x) and then use tan(3x)= tan(2x+ x).
Last edited by mr fantastic; Mar 1st 2009 at 06:16 PM.
Reason: Fixed some latex, deleted comments about original thread title (there's a new title now ....)