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\mathrm{US}=\sum_{\mathrm{i}=1}^{\mathrm{n}} \mathrm{f}\left(\mathrm{M}_{\mathrm{i}}\right) \Delta \mathrm{x}=\sum_{\mathrm{i}=1}^{\mathrm{n}}\left\{\frac{2}{\mathrm{n}} \mathrm{i}\right\}^{3} \frac{2}{\mathrm{n}}=\frac{2}{\mathrm{n}} \frac{8}{\mathrm{n}^{3}}\left\{\sum_{\mathrm{i}=1}^{\mathrm{n}} \mathrm{i}^{3}\right\}=\frac{16}{\mathrm{n}}\left\{\frac{1}{4} \mathrm{n}^{4}+\frac{1}{2} \mathrm{n}^{3}+\frac{3}{12} \mathrm{n}^{2}\right\}