trig
posted by MIK on .
Given sin a = 15/17 with a in Quadrant I and cos beta = 4/5 with beta in QuandrantIV, find the exact value of cos(a+beta)

sin a = 15/17 , with a in I, so cos a = 8/17
cos b = 4/5, with b in IV, then sin b = 3/5
cos(a+b) = cosa cosb  sina sinb
= (8/17)(4/5)  (15/17)(3/5)
= (32 + 45)/85
= 77/85