question

Here is a graph of  cos (x + pi/2):

and here is a graph of  - sin x :

WHat do you think?

We can use the angle sum identitiy for the cosine to  check this  ........

cos  (θ  + π/2) = 

cos θ cos π/2  -  sin  θ sin π/2  =

cos θ *  ( 0 )  - sin θ  * (1)  =

-sin θ

So...this is an  identity  !!!