– English translation by Henry Wadsworth Longfellowīabylonian mathematicians knew this for special cases before Greek mathematicians proved it.
– Dante's Paradiso, Canto 13, lines 100–102 It is generally attributed to Thales of Miletus, but it is sometimes attributed to Pythagoras. Thales's theorem is a special case of the inscribed angle theorem and is mentioned and proved as part of the 31st proposition in the third book of Euclid's Elements.
In geometry, Thales's theorem states that if A, B, and C are distinct points on a circle where the line AC is a diameter, the angle ∠ ABC is a right angle. Thales’ theorem: if AC is a diameter and B is a point on the diameter's circle, the angle ∠ ABC is a right angle. For the theorem sometimes called Thales' theorem and pertaining to similar triangles, see intercept theorem.
