用柱坐标,积分区域：0≤r≤z,0≤t≤2π,1≤z≤2.
∫∫∫z^2dxdydz=∫z^2dz∫dt∫rdr
=∫z^2dz∫dt(z^2/2)
=π∫z^4dz=π[z^5/5]=31π/5.