Evaluate the iterated integral. 2π 0 y 0 x 9 cos (x + y + z) dz dx dy 0 Step 1 We start with the innermost integral. When integrating with respect to z, the other variables x and y are considered constants, and so x 9 cos (x + y + z) dz 0 = 9 $$sin(x+y+z) x 0 = 9 $$sin(2x+y)−sin(x+y) .