The medial triangle or midpoint triangle of a triangle ABC is the triangle with vertices at the midpoints of the triangle's sides AB, AC and BC. It is the n=3 case of the midpoint polygon of a polygon with n sides. The medial triangle is not the same thing as the median triangle, which is the triangle whose sides have the same lengths as the medians of ABC.
The medial triangle can also be viewed as the image of triangle ABC transformed by a homothety centered at the centroid with ratio -1/2. Hence, the medial triangle is inversely similar and shares the same centroid and medians with triangle ABC. It also follows from this that the perimeter of the medial triangle equals the semiperimeter of triangle ABC, and that the area is one quarter of the area of triangle ABC. Furthermore, the four triangles that the original triangle is subdivided into by the medial triangle all all mutually congruent by SSS, so their areas are equal and thus the area of each is 1/4 the area of the original triangle.:p.177
Note that the orthocenter of the medial triangle coincides with the circumcenter of triangle ABC. This fact provides a tool for proving collinearity of the circumcenter, centroid and orthocenter. The medial triangle is the pedal triangle of the circumcenter. The nine-point circle circumscribes the medial triangle, and so the nine-point center is the circumcenter of the medial triangle.
Let a = |BC|, b = |CA|, c = |AB| be the sidelengths of triangle ABC. Trilinear coordinates for the vertices of the medial triangle are given by
- X = 0 : 1/b : 1/c
- Y = 1/a : 0 : 1/c
- Z = 1/a : 1/b : 0
If XYZ is the medial triangle of ABC, then ABC is the anticomplementary triangle or antimedial triangle of XYZ. The anticomplementary triangle of ABC is formed by three lines parallel to the sides of ABC: the parallel to AB through C, the parallel to AC through B, and the parallel to BC through A.
Trilinear coordinates for the vertices of the anticomplementary triangle, X'Y'Z', are given by
- X' = −1/a : 1/b : 1/c
- Y' = 1/a : −1/b : 1/c
- Z' = 1/a : 1/b : −1/c
The name "anticomplementary triangle" corresponds to the fact that its vertices are the anticomplements of the vertices A, B, C of the reference triangle. The vertices of the medial triangle are the complements of A, B, C.
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