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Thales’ rectangle
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Thales' rectangle
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Thales' rectangle
Thales' rectangle Diagram of Thales' rectangle, which is a proof that any angle inscribed in a semicircle is a right angle (90°).

Thales’ rectangle

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Thales of Miletus flourished about 600 bce and is credited with many of the earliest known geometric proofs. In particular, he has been credited with proving the following five theorems: (1) a circle is bisected by any diameter; (2) the base angles of an isosceles triangle are equal; (3) the opposite (“vertical”) angles formed by the intersection of two lines are equal; (4) two triangles are congruent (of equal shape and size) if two angles and a side are equal; and (5) any angle inscribed in a semicircle is a right angle (90°). Although none of Thales’ original proofs survives, ...(100 of 337 words)

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