1. ## Prove the identity

cos 4 theta - sin 4 theta = cos2 theta

2. ## Re: Prove the identity

Is the right hand side cos (2 theta) or cos^2(theta)? Since you used exponents on the left I presume it is the former. In that case, use the fact that cos(2 theta)= 1- 2sin^2(theta) and that cos^2(theta)= 1- sin^2(theta).

3. ## Re: Prove the identity

Originally Posted by Bonganitedd
cos 4 theta - sin 4 theta = cos2 theta
I'd treat the LHS as the difference of 2 squares.