A Riemann sum is an approximation to the integral, that is, an approximation using rectangles to the area mentioned above. The line segment from to is split into subsegments which form the bases of these rectangles, and the corresponding heights are determined by the value of at some point between the endpoints of the subsegment. The division of the segment into subsegments is called a partition. For the sake of convenience we will assume here that the subsegments are of equal width, although this is not strictly necessary.
The Riemann Sum is then given by the general formula:
There are five main types of Riemann Sums, depending on which point is chosen to determine the height:
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Right Sum: the right endpoint of the subsegment
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Left Sum: the left endpoint of the subsegment
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Middle Sum: the point half way between the left and right endpoints
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Lower Sum: any point such that is minimal
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Upper Sum: any point such that is maximal
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