原式=∮（x²+y²）ds+2∮zds=2/3∮(x²+y²+z²)ds+2/3∮(x+y+z)ds (用到轮换对称性）=2/3∮R²ds+2/3∮0ds (利用曲线积分可将曲线方程代入）=2/3 2πR=4/3 πR
