Proclus' variation of Euclid's proof proceeds as follows: The proof relies heavily on what is today called side-angle-side, the previous proposition in the Elements. One plausible explanation, given by Proclus, is that the second conclusion can be used in possible objections to the proofs of later propositions where Euclid does not cover every case. There has been much speculation and debate as to why Euclid added the second conclusion to the theorem, given that it makes the proof more complicated. But, as Euclid's commentator Proclus points out, Euclid never uses the second conclusion and his proof can be simplified somewhat by drawing the auxiliary lines to the sides of the triangle instead, the rest of the proof proceeding in more or less the same way. Euclid's proof involves drawing auxiliary lines to these extensions. Proofs Įuclid's Elements Book 1 proposition 5 the pons asinorumĮuclid's statement of the pons asinorum includes a second conclusion that if the equal sides of the triangle are extended below the base, then the angles between the extensions and the base are also equal. In fact, Marvin Minsky recounts that he had rediscovered the Pappus proof (which he was not aware of) by simulating what a mechanical theorem prover might do. Ī persistent piece of mathematical folklore claims that an artificial intelligence program discovered an original and more elegant proof of this theorem. Its first known usage in this context was in 1645. Pons asinorum is also used metaphorically for a problem or challenge which acts as a test of critical thinking, referring to the "ass' bridge's" ability to separate capable and incapable reasoners. The term is also applied to the Pythagorean theorem. Its converse is also true: if two angles of a triangle are equal, then the sides opposite them are also equal. This statement is Proposition 5 of Book 1 in Euclid's Elements, and is also known as the isosceles triangle theorem. In geometry, the statement that the angles opposite the equal sides of an isosceles triangle are themselves equal is known as the pons asinorum ( Latin:, English: / ˈ p ɒ n z ˌ æ s ɪ ˈ n ɔːr ə m/ PONZ ass-i- NOR-əm), typically translated as "bridge of asses". The pons asinorum in Byrne's edition of the Elements showing part of Euclid's proof.
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