# Geometric Proof of the Tetrahedral Number Formula

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The tetrahedral numbers 1, 4, 10, 20, 35, are the sums of the triangular numbers,

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Contributed by: Jim Delany (March 2011)

Open content licensed under CC BY-NC-SA

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This idea was suggested by a remark in Jeffrey Stopple's *A Primer of Analytic Number Theory: From Pythagoras to Riemann*, New York: Cambridge University Press, 2003.

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