where f is the function to integrate, a and b are the limits of the integration and {x1, x2,...,xn−1, xn} are a sequence of points on the x-axis with x1 = a and xn = b. x is the width of the interval...

where f is the function to integrate, a and b are the limits of the integration and {x1, x2,...,xn−1, xn} are a sequence of points on the x-axis with x1 = a and xn = b.

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x is the width of the interval between successive points. See the figure below for an illustration