1. use the Pythagorean identities and in that order on the left side.
They might need a slight rearrangement before you use them.
2. use the sum identity plus the fact that .
you'll also need the double angle identity for cosine
K I'm not really sure how to show what I mean without solving it. This a long way but I thought I show you a different way to work the problem.
Another way to see if the RHS is equal to the LHS is