"riemann-sum" is referenced in the pages listed below.

Riemann sums that use the left or right endpoints on the intervals can be used to find the height of the rectangles. On this page we explore the midpoint method uses a point in the middle of the interval to find the height of the rectangle, and the trapezoid method that uses a trapezoid instead of a rectangle to approximate the area of each interval. Interactive calculus applet.
In calculus, a Riemann sum is a method for approximating the total area underneath a curve on a graph, otherwise known as an integral. It may also be used to define the integration operation. This page explores this idea with an interactive calculus applet. Interactive calculus applet.