1. The problem statement, all variables and given/known data I was working on a problem in my book, and I didn't know how to answer the question. So I looked at the solution manual. and it had this http://imgur.com/VF4yl I do not know where the (cosx + 1) on the bottom came from. Any ideas? Please explain. 2. Relevant equations sin(x+y)= sinx cosy + cosx siny 3. The attempt at a solution I get the top part though, you factor out the 2, multiply the cos and sin then it becomes sin(x+y) sorta thing. Ok, nevermind, I am sorry, I didn't realise they factored out the cosx-1 and then multiplied by (cosx +1)/(cosx+1) So I got it now.