1. The problem statement, all variables and given/known data Derive a formula for cos 3a which would only involve cosine on the right side. 2. Relevant equations cos2x = cos(x)^2 - sin(x)^2 cos2x = 2cos(x)^2 - 1 cos2x = 1-2sin(x)^2 3. The attempt at a solution cos(3a) = cos(2a + a) = cos2acosa + sin2acosa = (2cos(a)^2 - 1)cosa - (2sinacosa)cosa = 2cos(a)^3 - cosa - 2sina cos(a)^2 Am I on the right path? Or should I just continue to do keep doing this?