2. Find cos(a + b) if sin(a)= 8/17 where a is in the first quadrant and tan(b)= -7/24 where b is in the second quadrant.

This is what I got for number 1:

sinα = 5/13 cos²α = 1-sin²α = 144/169 Since α is in the second quadrant, cosα = -√(144/169) = -12/13.

cosβ = -7/25 sin²β = 1-cos²β = 576/625 Since β is in the third quadrant, sinβ = -√(576/625) = -24/25

I just do not know how to do the sum formula for sine?

...and I am not sure how to do number 2?

Please explain. Thank you!