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\sum_{i=1}^{n} f\left(c_{i}\right) \cdot \frac{1}{n}=\sum_{i=1}^{n} f\left(c_{i}\right) \cdot \frac{b-a}{n} \cdot \frac{1}{b-a}=\frac{1}{b-a} \sum_{i=1}^{n} f\left(c_{i}\right) \cdot \frac{b-a}{n}=\frac{1}{b-a} \sum_{i=1}^{n} f\left(c_{i}\right) \cdot \Delta x