In geometry, the Exeter point is a special point associated with a plane triangle. The Exeter point is a triangle center and is designated as the center X(22) in Clark Kimberling's Encyclopedia of Triangle Centers. This was discovered in a computers-in-mathematics workshop at Phillips Exeter Academy in 1986. This is one of the recent triangle centers, having been discovered only in 1986, unlike the classical triangle centers like centroid, incenter, and Steiner point.
- Let ABC be any given triangle. Let the medians through the vertices A, B, C meet the circumcircle of triangle ABC at A' , B' and C' respectively. Let DEF be the triangle formed by the tangents at A, B, and C to the circumcircle of triangle 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' and FC' are concurrent. The point of concurrence is the Exeter point of triangle ABC.
The trilinear coordinates of the Exeter point are
- ( a ( b4 + c4 − a4 ), b ( c4 + a4 − b4 ), c ( a4 + b4 − c4 ) ).
- The Exeter point of triangle ABC lies on the Euler line (the line passing through the centroid, the orthocenter and the circumcenter) of triangle ABC.
- Kimberling, Clark. "Encyclopedia of Triangle Centers: X(22)". Retrieved 24 May 2012.
- Kimberling, Clark. "Exeter Point". Retrieved 24 May 2012.
- Kimberling, Clark. "Triangle centers". Retrieved 24 May 2012.
- Weisstein, Eric W. "Exeter Point". From MathWorld--A Wolfram Web Resource. Retrieved 24 May 2012.