n2\mathrm{n^2}n2
n(n+1)2\frac{\mathrm{n}(\mathrm{n}+1)}{2}2n(n+1)
{n(n+1)2}2\left\{\frac{\mathrm{n}(\mathrm{n}+1)}{2}\right\}^{2}{2n(n+1)}2
n(n+1)(2n+1)6\frac{\mathrm{n}(\mathrm{n}+1)(2 \mathrm{n}+1)}{6}6n(n+1)(2n+1)
