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Right triangle inscribed in a circle

Pamphila says that, having learnt geometry from the Egyptians, Thales was the first to inscribe in a circle a right-angled triangle, whereupon he sacrificed an ox. This is sometimes cited as history's first mathematical discovery. Due to the variations among testimonies, such as the story of the ox sacrifice being accredited to Pythagoras upon discovery of the Pythagorean theorem rather than Thales, some historians (such as D. R. Dicks) question whether such anecdotes have any historical worth whatsoever.

It is believed the Babylonians knew the theorem for special cases. The theorem is mentioned and proved as part of the 31st proposition in the third book of Euclid 's Elements. Dante's Paradiso refers to Thales's theorem in the course of a speech.