The set of Stewart is a set of Euclidean geometry , the one in the description of the geometry triangle is used. It can be used to calculate the length of a line through the corner of a triangle to the opposite side. It was drawn up in 1746 by the Scottish mathematician Matthew Stewart (although it was probably already known to Archimedes ).
A triangle is given (see picture) with the defining corner points A, B and C and the side lengths
; and .
Further, let M be a point on the line with
; and .
The Stewart's theorem then says:
(1)
Is the fraction with referred to, then (with )
and ,
and the sentence can also be formulated as follows:
(2)
Applications
Heron's important theorem for calculating the area of a triangle from its side lengths follows directly from Stewart's theorem. Stewart's theorem was also generalized for application to simplexes and tetrahedra by the Dutch mathematician Oene Bottema .
The Stewart's theorem includes the Pythagorean theorem . In the special case and it says:
One may without loss of generality assume that the triangle (see Figure), a geometrical figure of the complex plane represents and, in particular , the straight line with the real axis coincides at the same time applies, that is, the corner point in the upper half-plane is located. Otherwise, this situation can always be created by using suitably chosen plane congruence maps. Since congruent figures always have the same size relationships, it is sufficient to prove the theorem for this special case.
The following calculations to prove the theorem can then be made in three steps.
and further using the real part function and taking into account the fact that and :
In the penultimate equation, one multiplies left and right with , in the last equation left and right with , forms the sum of the respective left and right terms and, as it disappears, the following sum is obtained :
N. Altshiller-Court: Stewart's Theorem . In: College Geometry: A Second Course in Plane Geometry for Colleges and Normal Schools . 2nd ed.Barnes and Noble, 1952
O. Bottema: An extension of Stewart's formula . In: Elements of Mathematics , 34/1979, pp. 138–140, ( ISSN 0013-6018 )
O. Bottema: De formule van Stewart voor een viervlak . In: Nieuw Tijdschrift voor Wiskunde , 68 / 1980–81, pp. 79–83,
György Hajós : Introduction to Geometry . BG Teubner Verlag, Leipzig (Hungarian: Bevezetés A Geometriába . Translated by G. Eisenreich [Leipzig, also editing]).
Helmut Karzel , Hans-Joachim Kroll: History of Geometry since Hilbert . Scientific Book Society, Darmstadt 1988, ISBN 3-534-08524-8 .
Individual evidence
↑ Helmut Karzel , Hans-Joachim Kroll: History of geometry since Hilbert . Scientific Book Society, Darmstadt 1988, ISBN 3-534-08524-8 , pp.96 .
^ Based on Helmut Karzel , Hans-Joachim Kroll: History of Geometry since Hilbert . Scientific Book Society, Darmstadt 1988, ISBN 3-534-08524-8 , pp.384 . where, however, the proof is purely vectorial without the use of complex numbers.