The Riemann Sums applet is exceptional in its straightforward design, its ease of use, and its benefits for first semester calculus students. After a short learning curve for entering functions correctly (the syntax is easy for anyone with experience with math functions in a command line computer algebra system or in a programming environment), attention can be devoted entirely to exploration of the math concepts.
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Philippe B. Laval, "Riemann Sums - Editorial Review," Convergence (June 2004)