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\Delta x=\frac{2-0}{n}=\frac{2}{n} \cdot m_{i}=\frac{2}{n}(i-1) \text { and } M_{i}=\frac{2}{n} i \text { so } f\left(m_{i}\right)=\left\{\frac{2}{n}(i-1)\right\}^{3} \text { and } f\left(M_{i}\right)=\left\{\frac{2}{n} i\right\}^{3}