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Geometric representation of the square pyramidal number using cubes, instead of sferes, is more useful.
George Polya, in his book "Mathematical discovery", Volume 2, 1968, presents this solution saying it "rained from the sky", as obtained algebraically with a trick, like a rabbit drawn out from the hat.
Returning to the introduction to this talk page, seems that this geometric derivation is a little more ''direct'' than the algebraic presented in the article.
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