7/10. 用高斯公式得 I = ∫∫∫<Ω>3dv = 4πa^3.6/10. 用高斯公式得 I = ∫∫∫<Ω>(y+x)dv = ∫<0, a>dz∫<0, a>dy∫<0, a>(x+y)dx= ∫<0, a>dz∫<0, a>dy[x^2/2 + yx]<0, a>= ∫<0, a>dz∫<0, a>(a^2/2 + ay)dy= a[a^2y/2 + ay^2/2]<0, a> = a^4
