Find the exact value of the trig function below.

Question

 

cos(x+y) if sinx=12/13, cosx=5/13, siny=8/17, and cosy=15/17

Answer 1

As Michael pointed out, this is based on a trig identity you should know.  I recommend printing out a list of them to keep handy while you study (or bookmark the page in your textbook).  The one in question is a sum formula:

 

cos(x+y) = cos(x)cos(y) - sin(x)sin(y)

 

You have all the values on the right side of the formula, so all you need to do is substitute in to calculate the expression on the left.