This is given by the formula:
$$\sum_{k=1}^{n} f\left(x_{k}^{*}\right) \Delta x_{k}$$
where $f\left(x_{k}^{*}\right)$ is the value of the function at some point $x_{k}^{*}$ in the $k$-th subinterval, and $\Delta x_{k}$ is the width of the $k$-th subinterval.
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