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Steiner-Lehmus theorem




Any triangle that has two equal angle bisectors (each measured from a vertex to the opposite sides) is an isosceles triangle. In 1840, a Berlin professor Ludolph Lehmus wondered if this statement is true, given that it is the inverse of the already proven rule: If a triangle isosceles then two of its internal bisectors are equal. He put the problem to Jakob Steiner who was quickly able to show its validity. Shortly after, Lehmus himself found a neater proof and it is has since become a favorite pastime of geometry hobbyists to search for still simpler proofs of the theorem.


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   • GEOMETRY