Let $f(x)$ be a real-valued function on $[a,b]$. To give a Riemann sum of $f$ on $[a,b]$, we follow two steps:
Give a partition $a=x_0<x_1<\cdots<x_n=b$ of $[a,b]$.
Choose sample points $x_i^*$ in each subinterval $[x_{i},x_{i+1}]$ for $i=0,1,\ldots, n-1$.
Then, a Riemann sum is
\(f(x_0^\*)(x_1-x_0)+f(x_1^\*)(x_2-x_1)+\cdots+f(x_{n-1}^\*)(x_{n}-x_{n-1})=\sum_{i=0}^{n-1}f(x_i^\*)(x_i-x_{i-1}).\)
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