Exeter point

In geometry, the Exeter point is a special point associated with a plane triangle. It is a triangle center and is designated as X(22) ^[1] in Clark Kimberling's Encyclopedia of Triangle Centers. This was discovered in a computers-in-mathematics workshop at Phillips Exeter Academy in 1986. ^[2] This is one of the recent triangle centers, unlike the classical triangle centers like centroid, incenter, and Steiner point. ^[3]

Definition

  Reference triangle △ABC

   Medians of △ABC; concur at centroid

   Circumcircle of △ABC

  Triangle △A'B'C' formed by intersection of medians with circumcircle

   Tangential triangle △DEF of △ABC

  Lines joining vertices of △DEF and △A'B'C' ; concur at Exeter point

The Exeter point is defined as follows. ^{[2] [4]}

Let △ABC be any given triangle. Let the medians through the vertices A, B, C meet the circumcircle of △ABC at A', B', C' respectively. Let △DEF be the triangle formed by the tangents at A, B, C to the circumcircle of △ABC. (Let D be the vertex opposite to the side formed by the tangent at the vertex A, E be the vertex opposite to the side formed by the tangent at the vertex B, and F be the vertex opposite to the side formed by the tangent at the vertex C.) The lines through DA', EB', FC' are concurrent. The point of concurrence is the Exeter point of △ABC.

Put succinctly, the Exeter point is the perspector of the circummedial triangle and the tangential triangle.

Trilinear coordinates

The trilinear coordinates of the Exeter point are

$${\displaystyle a(b^{4}+c^{4}-a^{4}):b(c^{4}+a^{4}-b^{4}):c(a^{4}+b^{4}-c^{4})}$$

Properties

• The Exeter point of triangle ABC lies on the Euler line (the line passing through the centroid, the orthocenter, the de Longchamps point, the Euler centre and the circumcenter) of triangle ABC. So there are 6 points collinear over one only line.

References

1. Kimberling, Clark. "Encyclopedia of Triangle Centers: X(22)" . Retrieved 24 May 2012.
2. Kimberling, Clark. "Exeter Point" . Retrieved 24 May 2012.
3. Kimberling, Clark. "Triangle centers" . Retrieved 24 May 2012.
4. Weisstein, Eric W. "Exeter Point". From MathWorld--A Wolfram Web Resource. Retrieved 24 May 2012.