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). 4 cos s--and sin t =-, s and t in quadrant 1 cos (s + t)+] (Simplify your answer, including any radicals. Use integers or fractions for any numbers in the expression.)

Solution

cos s = 4/5 ; sins = 3/5

and sint = 1/5 ; cost = sqrt(24)/5

base = 4

cos(s+t) = coss*cost - sins*sint

=(4/5)(sqrt(24)/5) - (3/5)(1/5)

= [ 8 sqrt 6 - 3] / 25