1. The problem statement, all variables and given/known data Picture has been uploaded with question. 2. Relevant equations 3. The attempt at a solution I have literally never seen anything like this in my life. I'm in mathematical physics. I looked up de Moivre's formula and I guess this comes from a course in complex variables? I don't know why this is a homework question given that nobody in my class has actually taken it, but whatever. I need to be able to prove this. Can you please help me out, where should I start? I am familiar with geometric series and I guess I can reference De Moivre's formula. That's where I'm at. My initial thought was that this had something to do with a taylor expansion, but I know cos(x)'s taylor expansion and I don't think that has anything to do with this.