Answer:
steps below
Step-by-step explanation:
a,b in 2nd quadrant: sine is positive and cosine is negative
sin a = 3/4 cos a = -√7/4
cos b = -12/13 sin b = 5/12
sin(a+b) = sin a cos b + cos a sin b
= 3/4*(-12/13) + (-√7/4)*(5/12)
= - 9/13 - (5√7/48)
= -0.9679
cos(a-b) = cos a cos b + sin a sin b (can you try yourself?)
