Assume that 0 < x < pi/2 and 0 < y < pi/2. Find the exact value of cos(x - y) if cos(x) = 3/5 and cos(y) = 4/5.
draw a triangle to find sin(x) and sin(y)
(little drawing thing)
sin(x) = 4/5, sin(y) = 3/5 from the picture
cos(x-y) = cos(x)cos(y) + sin(x)sin(y)= (3/5)(4/5) + (4/5)(3/5) = 12/25 + 12/25 = 24/25
