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Fig 1. Construction of the first isogonic center, X(13). When no angle of the triangle exceeds 120°, this point is the Fermat point. In Euclidean geometry, the Fermat point of a triangle, also called the Torricelli point or Fermat–Torricelli point, is a point such that the sum of the three distances from each of the three vertices of the triangle to the point is the smallest possible [1] or ...
A point in the interior of a triangle is the center of an inellipse of the triangle if and only if the point lies in the interior of the triangle whose vertices lie at the midpoints of the original triangle's sides. [3]: p.139 For a given point inside that medial triangle, the inellipse with its center at that point is unique. [3]: p.142
The equation in trilinear coordinates x, y, z of any circumconic of a triangle is [1]: p. 192 l y z + m z x + n x y = 0. {\displaystyle lyz+mzx+nxy=0.} If the parameters l, m, n respectively equal the side lengths a, b, c (or the sines of the angles opposite them) then the equation gives the circumcircle .
The triangle angle sum theorem states that the sum of the three angles of any triangle, in this case angles α, β, and γ, will always equal 180 degrees. The Pythagorean theorem states that the sum of the areas of the two squares on the legs ( a and b ) of a right triangle equals the area of the square on the hypotenuse ( c ).
For example, a 7-simplex is (1,1) 8 = (1,2,1) 4 = (1,4,6,4,1) 2 = (1,8,28,56,70,56,28,8,1). The number of 1-faces (edges) of the n -simplex is the n -th triangle number , the number of 2-faces of the n -simplex is the ( n − 1) th tetrahedron number , the number of 3-faces of the n -simplex is the ( n − 2) th 5-cell number, and so on.
A triangle center P is said to be a major triangle center if the trilinear coordinates of P can be expressed in the form (): (): where is a function of the angle X alone and does not depend on the other angles or on the side lengths.
The center of the incircle is a triangle center called the triangle's incenter. [1] An excircle or escribed circle [2] of the triangle is a circle lying outside the triangle, tangent to one of its sides and tangent to the extensions of the other two. Every triangle has three distinct excircles, each tangent to one of the triangle's sides. [3]
The circumcenter of a tetrahedron can be found as intersection of three bisector planes. A bisector plane is defined as the plane centered on, and orthogonal to an edge of the tetrahedron. With this definition, the circumcenter C of a tetrahedron with vertices x 0, x 1, x 2, x 3 can be formulated as matrix-vector product: [35]