Thébault's theorem
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Thébault's theorem is the name given variously to one of the geometry problems proposed by the French mathematician Victor Thébault, individually known as Thébault's problem I, II, and III.
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[edit] Thébault's problem I
Given any parallelogram, construct on its sides four squares external to the parallelogram. The quadrilateral formed by joining the centers of those four squares is a square.
It is a special case of van Aubel's theorem.
[edit] Thébault's problem II
Given a square, construct equilateral triangles on two adjacent edges, either both inside or both outside the square. Then the triangle formed by joining the vertex of the square distant from both triangles and the vertices of the triangles distant from the square is equilateral.
[edit] Thébault's problem III
Given any triangle ABC, and any point M on BC, construct the incircle and circumcircle of the triangle. Then construct two additional circles, each tangent to AM, BC, and to the circumcircle. Then their centers and the center of the incircle are colinear.
This third problem of Thébault is the most difficult to prove. It was published in the American Mathematical Monthly in 1938, but was not proved until 1973, by the Dutch mathematician H. Streefkerk. 2003, Ayme discovered that Y. Sawayama solved this problem in 1905.
