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1. Lower Sum. "Lower Sum". From MathWorld-A Wolfram Web Resource. Contributed by: Weisstein, Eric W. (URL)
2. Upper Sum. "Upper Sum". From MathWorld-A Wolfram Web Resource. Contributed by: Weisstein, Eric W. (URL)
10. Disk. "The Disk Method". From The Wolfram Demonstrations Project.Contributed by: Lyle Cochran. (URL)
16. Marsden-Tromba. "Vector Calculus". W.H.Freeman & Co.