I know there are many ancient theorems like the Euclid's theoems concerning geomentry. The Pithagorean theorem is also very old, maybe known by Sumerians. Does someone knows what has been the first theorem ever discovered? Many thanks.
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The first mathematicians considered was Thales but the first theorem proved was a a little bit self evident but the important was that he wrote down a proof.
That was the theorem of the opposite angles [http://www.icoachmath.com/math_dictionary/Opposite_Angles.html].
Though there undoubtedly existed many mathematical truths in even prehistoric times that people knew about, it's not exactly clear that any theorems existed for prehistoric people. For any given mathematical truth, you don't end up having a theorem until there exists a proof of that theorem. From what I understand of the history of ancient civilizations, you don't find the notion of proof in mathematics becoming prominent until the Greeks. William Dunham in Journey Through Genius attributes the first theorem, or equivalently a mathematical "truth with a proof", to Thales of Miletus, and it gets called Thales Theorem. It says that if points A, B, and C lie on the circumference of a circle, and if line AC cuts across the diameter of a circle, then angle ABC is a right angle. It does not seem that Thales proof currently exists, and it's not clear that it ever got written down in text.