# De Gua's theorem De Gua's theorem Tetrahedron with a right-angle corner in O In mathematics, De Gua's theorem is a three-dimensional analog of the Pythagorean theorem named after Jean Paul de Gua de Malves. It states that if a tetrahedron has a right-angle corner (like the corner of a cube), then the square of the area of the face opposite the right-angle corner is the sum of the squares of the areas of the other three faces: {displaystyle A_{ABC}^{2}=A_{color {blue}ABO}^{2}+A_{color {green}ACO}^{2}+A_{color {red}BCO}^{2}} Generalizations The Pythagorean theorem and de Gua's theorem are special cases (n = 2, 3) of a general theorem about n-simplices with a right-angle corner. This, in turn, is a special case of a yet more general theorem by Donald R. Conant and William A. Beyer, which can be stated as follows.

Let U be a measurable subset of a k-dimensional affine subspace of {displaystyle mathbb {R} ^{n}} (so {displaystyle kleq n} ). For any subset {displaystyle Isubseteq {1,ldots ,n}} with exactly k elements, let {displaystyle U_{I}} be the orthogonal projection of U onto the linear span of {displaystyle e_{i_{1}},ldots ,e_{i_{k}}} , where {displaystyle I={i_{1},ldots ,i_{k}}} and {displaystyle e_{1},ldots ,e_{n}} is the standard basis for {displaystyle mathbb {R} ^{n}} . Then {displaystyle operatorname {vol} _{k}^{2}(U)=sum _{I}operatorname {vol} _{k}^{2}(U_{I}),} where {displaystyle operatorname {vol} _{k}(U)} is the k-dimensional volume of U and the sum is over all subsets {displaystyle Isubseteq {1,ldots ,n}} with exactly k elements.

De Gua's theorem and its generalisation (above) to n-simplices with right-angle corners correspond to the special case where k = n−1 and U is an (n−1)-simplex in {displaystyle mathbb {R} ^{n}} with vertices on the co-ordinate axes. For example, suppose n = 3, k = 2 and U is the triangle {displaystyle triangle ABC} in {displaystyle mathbb {R} ^{3}} with vertices A, B and C lying on the {displaystyle x_{1}} -, {displaystyle x_{2}} - and {displaystyle x_{3}} -axes, respectively. The subsets {displaystyle I} of {displaystyle {1,2,3}} with exactly 2 elements are {displaystyle {2,3}} , {displaystyle {1,3}} and {displaystyle {1,2}} . By definition, {displaystyle U_{{2,3}}} is the orthogonal projection of {displaystyle U=triangle ABC} onto the {displaystyle x_{2}x_{3}} -plane, so {displaystyle U_{{2,3}}} is the triangle {displaystyle triangle OBC} with vertices O, B and C, where O is the origin of {displaystyle mathbb {R} ^{3}} . Similarly, {displaystyle U_{{1,3}}=triangle AOC} and {displaystyle U_{{1,2}}=triangle ABO} , so the Conant–Beyer theorem says {displaystyle operatorname {vol} _{2}^{2}(triangle ABC)=operatorname {vol} _{2}^{2}(triangle OBC)+operatorname {vol} _{2}^{2}(triangle AOC)+operatorname {vol} _{2}^{2}(triangle ABO),} which is de Gua's theorem.

The generalisation of de Gua's theorem to n-simplices with right-angle corners can also be obtained as a special case from the Cayley–Menger determinant formula.

History Jean Paul de Gua de Malves (1713–85) published the theorem in 1783, but around the same time a slightly more general version was published by another French mathematician, Charles de Tinseau d'Amondans (1746–1818), as well. However the theorem had also been known much earlier to Johann Faulhaber (1580–1635) and René Descartes (1596–1650). See also Vector area and projected area Bivector Notes ^ Donald R Conant & William A Beyer (Mar 1974). "Generalized Pythagorean Theorem". The American Mathematical Monthly. Mathematical Association of America. 81 (3): 262–265. doi:10.2307/2319528. JSTOR 2319528. ^ Weisstein, Eric W. "de Gua's theorem". MathWorld. ^ Howard Whitley Eves: Great Moments in Mathematics (before 1650). Mathematical Association of America, 1983, ISBN 9780883853108, S. 37 (excerpt, p. 37, at Google Books) References Weisstein, Eric W. "de Gua's theorem". MathWorld. Sergio A. Alvarez: Note on an n-dimensional Pythagorean theorem, Carnegie Mellon University. De Gua's Theorem, Pythagorean theorem in 3-D — Graphical illustration and related properties of the tetrahedron. Further reading Kheyfits, Alexander (2004). "The Theorem of Cosines for Pyramids". The College Mathematics Journal. Mathematical Association of America. 35 (5): 385–388. JSTOR 4146849. Proof of de Gua's theorem and of generalizations to arbitrary tetrahedra and to pyramids. Lévy-Leblond, Jean-Marc (2020). "The Theorem of Cosines for Pyramids". The Mathematical Intelligencer. SpringerLink. Application of de Gua's theorem for proving a special case of Heron's formula. Categories: Theorems in geometryEuclidean geometry

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