Help help trig trig help

Please note that for some reason this site loses any "+" signs that you put in the subject header for your question. I assume what you mean is this:


\cos((\frac {\pi} 2 -x)-y) = \sin(x+y)


This is a siimple use of the identities for \cos (a-b) and \sin (a+b) , and also use of \cos (\frac {\pi} 2 - x) = \sin x and \sin(\frac {\pi} 2 - x) = \cos x.

Alternatively if you recognize that \cos((\frac {\pi} 2 -x)-y) = \cos(\frac {\pi} 2 -(x+y)) and apply the identity for \cos (a-b) then it comes right out.