# Math Help - write tan10x in terms of tan5x (?!)

1. ## write tan10x in terms of tan5x (?!)

We are supposed to:

"write tan10x in terms of tan5x."

I'm stumped! I don't remember learning this!

2. Do you know the double angle formula for $\tan (2\theta )$:
$\tan (2\theta) = \frac{2 \tan \theta}{1 - \tan^{2} \theta}$
Here, imagine $\theta = 5x$