Saccheri-Legendre Theorem
This topic defines a Dedekind cut and proves the Dedekind Axiom implies the Archimedian Axiom.
After introducig the measure of a segment and an angle, the triangular inequality and the Saccheri-Legendre Theorem are proven.
Proposition (Saccheri-Legendre) The sum of the degree measures of the three angles of a triangle is less than or equal to
![saccheri legendre theorem _gr_1.gif]](pages/saccheri-legendre-theorem/Images/saccheri-legendre-theorem_gr_1.gif)
Proof. Assume, on the contrary, that the angle sum of
is greater than
say
where
is a positive number.
By the Equilvalent Angle Sum Proposition, replace
with another traingle that has the same angle sum as
but in which one of the angles has at most half the number of degrees as
Repeat the procedure to get another triangle that has the same angle sum as
and has an angle that is one-quarter the degree measure of
The Archimedian Principle for real numbers guarantees that if this process is repeated enough times, eventually a triangle that has angle sum
and with one angle with degree measure at most
Thus, the sum of the degrees measures of the other two angles will be greater than or equal to
contradicting the Two Angles In A Triangle Proposition.
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Cite this as: Saccheri Legendre Theorem Published by Library of Math -- Online math organized by subject into topics.
Written by Smith, David A.
http://www.libraryofmath.com/saccheri-legendre-theorem.html
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