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__notoc__ De Gua's theorem is a three-dimensional analog of the Pythagorean theorem and named for Jean Paul de Gua de Malves. 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. : ==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, which can be stated as follows.〔Theorem 9 of James G. Dowty (2014). Volumes of logistic regression models with applications to model selection. 〕 Let ''P'' be a ''k''-dimensional affine subspace of (so ) and let ''C'' be a compact subset of ''P''. For any subset with exactly ''k'' elements, let be the orthogonal projection of ''C'' onto the linear span of , where and is the standard basis for . Then : where is the ''k''-dimensional volume of ''C'' and the sum is over all subsets with exactly ''k'' elements. This theorem is essentially the inner-product-space version of Pythagoras’ theorem applied to the ''k''th exterior power of ''n''-dimensional Euclidean space. 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 ''C'' is an (''n''−1)-simplex in with vertices on the co-ordinate axes. 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「De Gua's theorem」の詳細全文を読む スポンサード リンク
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