The question is to solve the equation y'' + ω^2y = cos(ωt) I know you'll find the complementary and particular functions and add them together. Now I found the complementary function easily. r= +/- ωi and then plug into the general equation for complex numbers. The problem I have is with determining the particular function: When I do it, I get 0 = cos(ωt) The soltuion I guseed was Acos(ωt) + Bsin(ωt). I'm confused :(.