△S≈△Sn=f(i/n)△x=(i/n)^2△x
=2/n[(1/n)^2+(2/n)^2+…+(n/n)^2]
=8/n^3(n+1)(2n+1)/6
=8/3+4/n+4/3n^2
当n趋近于无穷大时,△Sn=8/3