n(n+1)(2n+1)2\mathrm { \frac{n(n+1)(2n+1)}{2}}2n(n+1)(2n+1)
{n(n+1)2}2\bigg\{\mathrm { \frac{n(n+1)}{2}}\bigg\}^2{2n(n+1)}2
n(n+1)2\mathrm { \frac{n(n+1)}{2}}2n(n+1)
n2\mathrm {n^2}n2
