Nagel point

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The Nagel point (blue, N) of a triangle (black). The red triangle is the extouch triangle, and the orange circles are the excircles
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 point associated with any triangle. In the plane of a triangle ABC with side lengths a = |BC|, b = |CA|, and c = |AB|, let TA, TB, and TA be the points in which the A-excircle meets line BC, the B-excircle meets line CA, and C-excircle meets line AB, respectively. The lines ATA, BTB, CTC 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 TA is to start at A and trace around triangle ABC half its perimeter, and similarly for TB and TC. Because of this construction, the Nagel point is sometimes also called the bisected perimeter point.

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[edit] Relation to other triangle centers

The Nagel point is the isotomic conjugate of the Gergonne point. The Nagel point, the incenter, and the centroid are collinear. The incenter is the Nagel point of the medial triangle (Anonymous 1896); equivalently, the Nagel point is the incenter of the anticomplementary triangle.

[edit] Trilinear coordinates

Trilinear coordinates of the Nagel point were given by Gallatly (1913) as

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

or, equivalently,

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

[edit] See also

[edit] References

  • Baptist, Peter (1987). "Historische Anmerkungen zu Gergonne- und Nagel-Punkt". Sudhoffs Archiv für Geschichte der Medizin und der Naturwissenschaften 71 (2): 230–233. MR0936136. 
  • Gallatly, William (1913). The Modern Geometry of the Triangle, 2nd ed., London: Hodgson, page 20. 

[edit] External links