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Thalesā€™ 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 (90Ā°).

Thalesā€™ rectangle

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History

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 (ā€œverticalā€) 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 (90Ā°).

Although none of Thalesā€™ original proofs survives, the English mathematician Thomas Heath (1861ā€“1940) proposed what is now known as Thalesā€™ rectangle as a proof of theorem 5 that would have been consistent with what was known in Thalesā€™ era.

Beginning with āˆ ACB inscribed in the semicircle with diameter AB, draw the line from C through the corresponding circleā€™s centre O such that it intersects the circle at D. Then complete the quadrilateral by drawing the lines AD and BD. First, note that the lines AO, BO, CO, and DO are equal because each is a radius, r, of the circle. Next, note that the vertical angles formed by the intersection of lines AB and CD form two sets of equal angles, as indicated by the tick marks. Applying a theorem known to Thales, the side-angle-side (SAS) theoremā€”two triangles are congruent if two sides and the included angle are equalā€”yields two sets of congruent triangles: ā–³AOD ā‰… ā–³BOC and ā–³DOB ā‰… ā–³COA. Since the triangles are congruent, their corresponding parts are equal: āˆ ADO = āˆ BCO, āˆ DAO = āˆ CBO, āˆ BDO = āˆ ACO, and so forth. Since all of these triangles are isosceles, their base angles are equal, which means that there are two sets of four angles that are equal, as indicated by the tick marks. Finally, since each angle of the quadrilateral has the same composition, the four quadrilateral angles must be equalā€”a result that is only possible for a rectangle. Therefore, āˆ ACB = 90Ā°.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Erik Gregersen.