When you read "The Elements" by Euclid you are reading a copy of a copy of a copy of a math textbook written 2000 years ago.

Book V was mostly based on the work of Eudoxus of Cnidus, who was a Pythagorean and so in it you have a proof of "The Pythagorean Theorem" which we attribute to Pythagoras.

I can imagine that this proof was first expressed in lectures and lessons and probably made famous by Pythagoras himself or one of his followers.

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