Answer:
cos(X+Y) = cos(55+35) = cos(90) = 0
Step-by-step explanation:
cos(X+Y) = cosXcosY - sinXsinY
cos(55+35) = cos(55)cos(35) - sin(55)sin(35)
cos(55+35) = cos(55)cos(35) - cos(90-55)cos(90-35)
cos(55+35) = cos(55)cos(35) - cos(35)cos(55)
cos(55+35) = 0
cos(90) = 0
