1. The problem statement, all variables and given/known data I have the answer I just dont know how he got it! I want to know how he got from the second line to the third line in the image below. 2. Relevant equations 3. The attempt at a solution I have used integration by part and the u substitution method. When substituting with u i am still left with the other terms in the other sine function. When integrating by parts, it just doesn't simplify.. Is there another method i don't know about? There doesn't seem to be any trig identities either.