integral of cos(x)sin(2x)
the answer is (-1/6)cos(3x)-.5cos(x)
Yeah, but what you don't know is how many problems I ran into that I couldn't integrate before I finally started remembering this trick!
You should try kalagota's method also. It's always good to practice trig integrals (since they are, in my opinion, the nastiest kind.)
-Dan