Find cosst and cosst coss15 and sint35 s and t are in quadra

Find cos(s+t) and cos(s-t)

coss=-1/5 and sint=3/5, s and t are in quadran II.

Thank you for your help!

cos(s+t)= cos s cost - sin s sin t

cos = -1/5

adjacent=-1 hypotenuse=5 therefore opposite= sqrt 24

sin s= sqrt 24/5

sin t= 3/5 opposite=-3 hypotenuse=-5 therefore adjacent = 4

cost= -4/5

cos(s+t)= (-1/5)(-4/5) - (sqrt 24/5)(3/5)= 4/25-3sqrt24/25

=(4-3sqrt24)/25

cos(s-t)= cos cost +sin s sint= (4+3sqrt24)/25