This is Nadene (student), and I'm having difficulty trying to solve a precal question (high school). The question is: Prove the identity. cos [x + (ypi/2)] = sin (x+y) A hint was also provided which is: "Apply cos (alpha + beta) first then within that apply cos (alphabeta)" Any help on solving this will be greatly appreciated. Thank you 

Hi Nadene, I can see three ways to prove this identity and the way suggested by the hint is, in my opinion, the worst of the three. The proof, suggested by the hint, goes as follows:
The second proof is simpler.
The third proof is, in my opinion, the easiest.


