Need Help Very Confused

Question

Find cos (a +b) if sin (a) = -8/17 where a is in the fourth quadrant and tan (b) = -20/21where b is in the second quadrant. 

Answer 1

cos a =15/17 (relying on Pythagorean identity)

sin b = 20/29 and cos b = -21/29.   It helps to draw the reference triangle and use Pythag. Theorem.

 

now you have everything needed in cos(a+b) = cos a cos b - sin a sin b.