感觉有点怀疑自己(￣▽￣)
2. M = ∫∫∫<Ω>(x+y)dv = ∫<0,π/2>dt∫<0, 1>(rcost+rsint)rdr∫dz= ∫<0,π/2>dt∫<0, 1>(cost+sint)r^2(1-r^2)dr= ∫<0,π/2>(cost+sint)dt∫<0, 1>(r^2-r^4)dr= (2/15)[sint-cost]<0,π/2> = 4/15
