Can you post solved problems using these conditions? We're on the Advanced Engineering Mathematics and reviewing our Differential Equations. My professor only introduces this topic and didn't went deeper. I'm troubled about the sign convention, e.g.: (D^2-9)y=-sin4x (D^2-a^2)y=sinbx yp=-sinbx/(a^2+b^2) a=3, b=4 yp=sin4x/(3^2+4^2) yp=sin4x/25 how does yp became positive? What are the formulae used... also for cosine condition? Thanks for your help!