Nagel point

The Nagel point (blue, N) of a triangle (black). The red triangle is the extouch triangle, and the orange circles are the excircles

In geometry, the Nagel point is a triangle center, one of the points associated with a given triangle whose definition does not depend on the placement or scale of the triangle. Given a triangle ABC, let T_A, T_B, and T_C be the extouch points in which the A-excircle meets line BC, the B-excircle meets line CA, and C-excircle meets line AB, respectively. The lines AT_A, BT_B, CT_C concur in the Nagel point N of triangle ABC. The Nagel point is named after Christian Heinrich von Nagel, a nineteenth-century German mathematician, who wrote about it in 1836.

Another construction of the point T_A is to start at A and trace around triangle ABC half its perimeter, and similarly for T_B and T_C. Because of this construction, the Nagel point is sometimes also called the bisected perimeter point, and the segments AT_A, BT_B, CT_C are called the triangle's splitters.

Relation to other triangle centers

The Nagel point is the isotomic conjugate of the Gergonne point. The Nagel point, the centroid, and the incenter are collinear on a line called the Nagel line. The incenter is the Nagel point of the medial triangle; equivalently, the Nagel point is the incenter of the anticomplementary triangle.

Trilinear coordinates

The trilinear coordinates of the Nagel point are as

$\csc^2(A/2)\,:\,\csc^2(B/2)\,:\,\csc^2(C/2)$

or, equivalently, in terms of the side lengths a = |BC|, b = |CA|, and c = |AB|,

$\frac{b + c - a}{a}\,:\,\frac{c + a - b}{b}\,:\,\frac{a + b - c}{c}.$

See also

• Mandart inellipse
• Trisected perimeter point

External links

• Nagel Point from Cut-the-knot
• Nagel Point, Clark Kimberling
• Weisstein, Eric W., "Nagel Point", MathWorld.
• Spieker Conic and generalization of Nagel line at Dynamic Geometry Sketches Generalizes Spieker circle and associated Nagel line.fr:Cercles inscrit et exinscrits d'un triangle#Point de Nagel