The anticomplementary triangle is the triangle which has a given triangle as its medial triangle. It is therefore the anticevian triangle with respect to the triangle centroid (Kimberling 1998, p. 156), and is in perspective with at .
It is the polar triangle of the Steiner circumellipse.
Its trilinear vertex matrix is
(1)
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or
(2)
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The sides of the anticomplementary triangle are 's exmedians and the vertices are the exmedian points of .
The circumcircle of the anticomplementary triangle is the anticomplementary circle.
The following table gives the centers of the anticomplementary triangle in terms of the centers of the reference triangle for Kimberling centers with .
The medial triangle of a triangle is similar to and its side lengths are
(3)
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(4)
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(5)
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This follows immediately by inspecting the construction of the medial triangle and noting that the three vertex triangles and central triangle each have sides of length , , and . Similarly, each of these triangles, including , have area
(6)
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where is the triangle area of .