find the exact value of cos(A+B)??

Question

if sinA= -5/13 in QIII, and secB=5/3 in QI

that's supposed to be a neg. in front of the 5/13... obviously

thanks a lot for your help!

Answer 1

cos(A+B) = cosAcosB - sinAsinB = (-12/13)(3/5) - (-5/13)(4/5) = -16/65