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The Pythagorean theorem is a relation in Euclidean geometry between the three sides of a right angled triangle. The theorem is named after and commonly attributed to the sixth century BC Greek philosopher and mathematician Pythagoras, although the facts of the theorem were known by Indian, Greek, Chinese and Babylonia mathematicians well before he lived.

The first known proof of the Pythagorean theorem can be found in Euclid's Elements.