A Geometric Proof of the Square Pyramidal Number Formula
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The pyramidal numbers 1, 5, 14, 30, … are the sums of the square numbers, 
. This Demonstration shows a geometric proof of the square pyramidal number formula, 
. In the figure the six pyramids of side length 
form an 
cuboid.
Contributed by: Okay Arik (January 2008)
Open content licensed under CC BY-NC-SA
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"A Geometric Proof of the Square Pyramidal Number Formula"
http://demonstrations.wolfram.com/AGeometricProofOfTheSquarePyramidalNumberFormula/
Wolfram Demonstrations Project
Published: January 25 2008
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A Geometric Proof of the Square Pyramidal Number Formula
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