Brocard point

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Brocard points are special points within a triangle. Henri Brocard was a French mathematician who lived from 1845 until 1922. He is best known for the following theorem.

In a triangle ABC with sides a, b, and c, there is exactly one point P such that the line segments AP, BP, and CP form the same angle, ω, with the respective sides c, a, and b, namely that $\angle PBC = \angle PCA = \angle PAB$ . Point P is called the first Brocard point of the triangle ABC, and the angle ω is called the Brocard angle of the triangle. The following appies to this angle:

cotω = cotα + cotβ + cotγ.

There is also a second Brocard point, Q, in triangle ABC such that line segments AQ, BQ, and CQ form equal angles with sides b, c, and a respectively. In other words, the equations $\angle QCB = \angle QBA = \angle QAC$ apply. Remarkably, this second Brocard point has the same Brocard angle as the first Brocard point. In other words angle $\angle PBC = \angle PCA = \angle PAB$ is the same as $\angle QCB = \angle QBA = \angle QAC$ .

The two Brocard points are closely related to one another; In fact, the difference between the first and the second depends on the order in which the angles of triangle ABC are taken. So for example, the first Brocard point of triangle ABC is the same as the second Brocard point of triangle ACB.

The two Brocard points of a triangle ABC are isogonal conjugates of each other.

1 Construction
2 Trilinears and the Brocard midpoint
3 Reference
4 External Links

[edit] Construction

The most elegant construction of the Brocard points goes as follows. In the following example the first Brocard point is presented, but the construction for the second Brocard point is very similar.

Intersect the perpendicular bisector of side AB with the perpendicular to line BC through point B. Draw a circle with the intersection point as its center so that the circle goes through point B. This circle will also go through point A and intersect line BC at point B. We similarly construct a circle through points C and B, which touches line CA at point C, and a circle through points A and C, which touches line AB at point A. These three circles have a common point – the first Brocard point of triangle ABC!

The three circles just constructed are also designated as epicycles of triangle ABC. The second Brocard point is constructed in similar fashion.

[edit] Trilinears and the Brocard midpoint

Homogeneous trilinear coordinates for the first and second Brocard points are c/b : a/c : b/a, and b/c : c/a : a/b, respectively. They are an example of a bicentric pair of points, but not triangle centers. Their midpoint, called the Brocard midpoint, has trilinears

sin(A+ω) : sin(B+ω) : sin(C+ω)

and is a triangle center. The third Brocard point, given by trilinears a^-3 : b^-3 : c^-3, or, equivalently, by

csc(A-ω) : csc(B-ω) : csc(C-ω),

is the Brocard midpoint of the anticomplementary triangle and is also the isotomic conjugate of the symmedian point.

[edit] Reference

Ross Honsberger, "The Brocard Points," Chapter 10 in Episodes in Nineteenth and Twentieth Century Euclidean Geometry, The Mathematical Association of America, Washington, D.C., 1995.