1. The problem statement, all variables and given/known data Establish the identity: 1+cos(2θ)+cos(4θ)+cos(6θ)=4cosθcos(2θ)cos(3θ) 2. Relevant equations cos(a)+cos(b)=2cos((a+b)/2)cos((a-b)/2) 3. The attempt at a solution I understand how to do a simple cos(+/-)cos problem according to the Sums as Products equations, but I am confused on how to handle adding 3 cosine functions plus 1. EDIT: cos(0)=1, so I split the problem into 2 additions problems, cos(0θ)+cos(2θ) and cos(4θ)+cos(6θ).