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 name "trilinear coordinates" is sometimes abbreviated to "trilinears".
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 triangle ABC. Given side lengths a, b, c we have:
Note that, in general, the incenter is not the same as the centroid; the centroid has barycentric coordinates 1 : 1 : 1 (these being proportional to actual signed areas of the triangles BGC, CGA, AGB, where G = centroid.)
The midpoint of, for example, side BC has trilinear coordinates in actual sideline distances for triangle area , which in arbitrarily specified relative distances simplifies to The coordinates in actual sideline distances of the foot of the altitude from A to BC are which in purely relative distances simplifies to :p. 96
Trilinear coordinates enable many algebraic methods in triangle geometry. For example, three points
equals zero. Thus if x:y:z is a variable point, the equation of a line through the points P and U is D = 0.:p. 23 From this, every straight line has a linear equation homogeneous in x, y, z. Every equation of the form lx+my+nz = 0 in real coefficients is a real straight line of finite points unless l : m: n is proportional to a : b : c, the side lengths, in which case we have the locus of points at infinity.:p. 40
The dual of this proposition is that the lines
Also, if the actual directed distances are used when evaluating the determinant of D, then the area of triangle PUX is KD, where K = abc/8?2 (and where ? is the area of triangle ABC, as above) if triangle PUX has the same orientation (clockwise or counterclockwise) as triangle ABC, and K = -abc/8?2 otherwise.
Two lines with trilinear equations and are parallel if and only if:p. 98,#xi
where a, b, c are the side lengths.
Thus two lines with trilinear equations and are perpendicular if and only if
The equation of a line with variable distances p, q, r from the vertices A, B, C whose opposite sides are a, b, c is:p. 97,#viii
The trilinears with the coordinate values a', b', c' being the actual perpendicular distances to the sides satisfy:p. 11
for triangle sides a, b, c and area . This can be seen in the figure at the top of this article, with interior point P partitioning triangle ABC into three triangles PBC, PCA, and PAB with respective areas (1/2)aa' , (1/2)bb' , and (1/2)cc' .
The distance d between two points with actual-distance trilinears ai : bi : ci is given by:p. 46
or in a more symmetric way
The distance d from a point a' : b' : c' , in trilinear coordinates of actual distances, to a straight line lx + my + nz = 0 is:p. 48
It has no linear terms and no constant term.
The equation of a circle of radius r having center at actual-distance coordinates (a', b', c' ) is:p.287
Each distinct circumconic has a center unique to itself. The equation in trilinear coordinates of the circumconic with center x' : y' : z' is:p. 203
with exactly one or three of the unspecified signs being negative.
Many cubic curves are easily represented using trilinear coordinates. For example, the pivotal self-isoconjugate cubic Z(U,P), as the locus of a point X such that the P-isoconjugate of X is on the line UX is given by the determinant equation
Among named cubics Z(U,P) are the following:
For any choice of trilinear coordinates x:y:z to locate a point, the actual distances of the point from the sidelines are given by a' = kx, b' = ky, c' = kz where k can be determined by the formula in which a, b, c are the respective sidelengths BC, CA, AB, and ? is the area of ABC.
A point with trilinear coordinates x : y : z has barycentric coordinates ax : by : cz where a, b, c are the sidelengths of the triangle. Conversely, a point with barycentrics ? : ? : ? has trilinear coordinates ?/a : ?/b : ?/c.
Given a reference triangle ABC, express the position of the vertex B in terms of an ordered pair of Cartesian coordinates and represent this algebraically as a vector B, using vertex C as the origin. Similarly define the position vector of vertex A as A. Then any point P associated with the reference triangle ABC can be defined in a Cartesian system as a vector P = k1A + k2B. If this point P has trilinear coordinates x : y : z then the conversion formula from the coefficients k1 and k2 in the Cartesian representation to the trilinear coordinates is, for side lengths a, b, c opposite vertices A, B, C,
and the conversion formula from the trilinear coordinates to the coefficients in the Cartesian representation is
More generally, if an arbitrary origin is chosen where the Cartesian coordinates of the vertices are known and represented by the vectors A, B and C and if the point P has trilinear coordinates x : y : z, then the Cartesian coordinates of P are the weighted average of the Cartesian coordinates of these vertices using the barycentric coordinates ax, by and cz as the weights. Hence the conversion formula from the trilinear coordinates x, y, z to the vector of Cartesian coordinates P of the point is given by
where the side lengths are |C - B| = a, |A - C| = b and |B - A| = c.