1. The problem statement, all variables and given/known data Simplify: cos(4θ) 2. Relevant equations cos(2θ)=2cos^2(θ)-1 sin(2θ)=2sinθcosθ 3. The attempt at a solution First, I broke it into cos(2θ+2θ). Then I expanded it and got cos(2θ)cos(2θ)-sin(2θ)sin(2θ). I then expanded that and got (4cos^4(θ)-4cos^2(θ)+1)-(4sin^2(θ)+4sin(θ)cos(θ)+cos^2). I was supposed to get 8cos^4(θ)-8cos^2(θ)+1. I see that I'm off but I don't know where I went wrong.