5.93 How to find trig identities?

use trigsubs, very useful command. For example

trigsubs(cos(theta)^3)
 

Gives

\[ [1/2\,\cos \left ( \theta \right ) +1/2\,\cos \left ( 2\,\theta \right ) \cos \left ( \theta \right ) ,1/4\,\cos \left ( 3\,\theta \right ) +3/4\,\cos \left ( \theta \right ) ] \]