1. The problem statement, all variables and given/known data [itex]\int dx/(cosx + cos3x)[/itex] 2. Relevant equations 3. The attempt at a solution Not sure if I'm doing this right. [itex]1/cosx + cos3x = 1/2cosx * cos2x = cosx/cosx * 2cosxcos2x = cosx/2cos^2xcosx = (1/2)*(cosx)/[(1-sin^2x)(1-2sin^2x)][/itex] then we let t = sinx and get [itex]\int dx/(cosx + cos3x) = 1/2 [1/(1-t^2)(1-2t^2)]dt[/itex] Any comments please..