TeX source:

\mathrm{US}=\sum_{i=1}^{\mathrm{n}} \mathrm{f}\left(\mathrm{M}_{\mathrm{i}}\right) \Delta \mathrm{x}=\sum_{\mathrm{i}=1}^{\mathrm{n}} \frac{4}{\mathrm{n}^{2}} \mathrm{i}^{2} \frac{2}{\mathrm{n}}=\frac{8}{\mathrm{n}^{3}}\left\{\sum_{\mathrm{i}=1}^{\mathrm{n}} \mathrm{i}^{2}\right\}