>>255
x=rcosθ　, y=rsinθ　とおく。

= ∫[r=0,1]∫[θ=-π/2,π/2] rcosθ*rdrdθ ∫[z=0,2]zdz
= ∫[r=0,1] r^2dr ∫[θ=-π/2,π/2] cosθdθ ∫[z=0,2]zdz
= (1/3) * 2 * 2
= 4/3