1. The problem statement, all variables and given/known data Use complex exponentials to prove 1 + acos(theta) + a^2cos(2theta) + a^3cos(3theta)... = (1 - acos(theta))/(1 - 2acos(theta) + a^2) 2. Relevant equations euler's e^itheta/2 +e^-itheta/2=2cos(2theta) 3. The attempt at a solution a^(n)cos(ntheta) = e^nitheta = e^-nitheta from there i got the series (a^n(e^itheta)^n)/2 now from here I think I setup the summation formula but this is where I get stuck. Any help is greatly apprecited.