区域为球体x² + y² + z² = a²?
∫∫∫Ω (x² + y²) dV
= 2∫∫∫Ω x² dV
= (2/3)∫∫∫Ω (x² + y² + z²) dV
= (2/3)∫(0→2π) dθ ∫(0→π) sinφ dφ ∫(0→a) r⁴ dr
= (2/3)(2π)(2)(a⁵/5)
= (8/15)πa⁵