Use the cosine of a sum and cosine of a difference identitie

Use the cosine of a sum and cosine of a difference identities to find cos(s + t) and cos(s - t) sin s = -4/5 and sin t 5/13, in quadrant III and t in quadrant I cos(s + 1) = cos (s - t) =

Solution

sin s = -4/5 ; cos s = -3/5

sin t = 5/13 ; cos t = 12/13

Cos(s + t) = cos s cos t sin s sin t = (-3/5)*(12/13) - (-4/5)*(5/13) = -0.246

cos(s - t) = cos s cos t + sin s sin t = (-3/5)*(12/13) + (-4/5)*(5/13) = -0.862