设∑为上半球面x2+y2+z2=a2d的外侧，则∫∫∑zdxdy=____．

设∑为上半球面x²+y²+z²=a²d的外侧，则∫∫_∑zdxdy=____．
【正确答案】：∫∫_∑zdxdy=∫∫_D√(a²-x²-y²)•[a/√(a²-x²-y²)]dxdy=(2/3)πa³