Question

Verify the identity: cos(x+pi/2) = -sin x
I think the point of this question is to use the sum/difference formula.

Answer 1

In general, any identity proof is done using the identities that have already been learned, This means you need to look in your book and see what you officially know. Then you apply this in such a way that either the left hand side eventually looks like the right, or the other way around, but you never work both sides.

Some examples: http://www.purplemath.com/modules/proving.htm

So what have you tried and where does it get stuck?