Trilinear coordinates

In geometry, the trilinear coordinates x : y : z of a point relative to a given triangle describe the relative directed distances from the three sidelines of the triangle. Trilinear coordinates are an example of homogeneous coordinates. The ratio x : y is the ratio of the perpendicular distances from the point to the sides (extended if necessary) opposite vertices A and B respectively; the ratio y : z is the ratio of the perpendicular distances from the point to the sidelines opposite vertices B and C respectively; and likewise for z : x and vertices C and A. In the diagram at right, the trilinear coordinates of the indicated interior point are the actual distances (a', b', c'), or equivalently in ratio form, ka' : kb' : kc' for any positive constant k. If a point is on a sideline of the reference triangle, its corresponding trilinear coordinate is 0. If an exterior point is on the opposite side of a sideline from the interior of the triangle, its trilinear coordinate associated with that sideline is negative. It is impossible for all three trilinear coordinates to be non-positive. The ratio notation x : y : z for trilinear coordinates is different from the ordered triple notation (a', b', c' ) for actual directed distances. Here each of x, y, and z has no meaning by itself; its ratio to one of the others does have meaning. Thus "comma notation" for trilinear coordinates should be avoided, because the notation (x, y, z), which means an ordered triple, does not allow, for example, (x, y, z) = (2x, 2y, 2z), whereas the "colon notation" does allow x : y : z = 2x : 2y : 2z. The trilinear coordinates of the incenter of a triangle △ABC are 1 : 1 : 1; that is, the (directed) distances from the incenter to the sidelines BC, CA, AB are proportional to the actual distances denoted by (r, r, r), where r is the inradius of △ABC. Given side lengths a, b, c we have: A = 1 : 0 : 0 B = 0 : 1 : 0 C = 0 : 0 : 1 incenter = 1 : 1 : 1 centroid = bc : ca : ab = 1/a : 1/b : 1/c = csc A : csc B : csc C. circumcenter = cos A : cos B : cos C.

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