1. The problem statement, all variables and given/known data I have to prove that 3A=(cos(2A)-cos(4A))/(sin(4A)-sin(2A)) 2. Relevant equations All of the trig identites. 3. The attempt at a solution Well, I know I can expand tan(3A) to sin(3A)/cos(3A). After this, I have no idea what to do. I do not know any way to simplify or expand the right side of the equation. If someone can help me out by just showing the me the first step, if possible, then I can try to solve the rest of the identity by myself. Thanks!