De Gua's theorem
The set of de Gua is a spatial analogue of the theorem of Pythagoras and according to Jean Paul de Gua de Malves (1713-1785) appointed who published it in 1783.
When a tetrahedron, a right-angled corner (such as a cube corner ) has, then the sum of squares of the areas of the surfaces adjacent to the right angle corner is equal to the squared surface area of the side opposite to the right angle surface.
The Pythagorean theorem and the theorem of de Gua are special cases ( n = 2.3 ) of a general theorem on n- simplexes with a " right-angled " corner.
The set was released at the same time in a somewhat more general form by the French mathematician Tinseau d' Amondans (1746-1818) and was even much earlier René Descartes (1596-1650) and John Faulhaber (1580-1635) was known.