用球坐标换元，x^2+y^2+z^2=r^2, dxdydz=r^2*sinφdrdθdφ,积分区域：r：0->1; θ:0->2π; φ:0->π/2原式=∫∫∫(x^2+y^2+z^2)dxdydz=∫dθ∫sinφdφ∫r^4dr=2π*1*1/5=2/5*π
