Symmedian

"Lemoine Point" redirects here. For other uses, see Lemoine Point (disambiguation).

File:Symmedians.png

A triangle with medians (blue), angle bisectors (green) and symmedians (red). The symmedians intersect in the symmedian point K, the angle bisectors in the incenter I and the medians in the centroid G.

In geometry, symmedians are three particular geometrical lines associated with every triangle. They are constructed by taking a median of the triangle (a line connecting a vertex with the midpoint of the opposite side), and reflecting the line over the corresponding angle bisector (the line through the same vertex that divides the angle there in half). The angle formed by the symmedian and the angle bisector has the same measure as the angle between the median and the angle bisector, but it is on the other side of the angle bisector.

The three symmedians meet at a triangle center called the symmedian point. Ross Honsberger called its existence "one of the crown jewels of modern geometry".

Isogonality

Many times in geometry, if we take three special lines through the vertices of a triangle, or cevians, then their reflections about the corresponding angle bisectors, called isogonal lines, will also have interesting properties. For instance, if three cevians of a triangle intersect at a point P, then their isogonal lines also intersect at a point, called the isogonal conjugate of P.

The symmedians illustrate this fact.

• In the diagram, the medians (in blue) intersect at the centroid G.
• Because the symmedians (in red) are isogonal to the medians, the symmedians also intersect at a single point, K.

This point is called the triangle's symmedian point, or alternatively the Lemoine point or Grebe point.

The green lines are the angle bisectors; the symmedians and medians are symmetric about the angle bisectors (hence the name "symmedian.")

Symmedian point

In the Encyclopedia of Triangle Centers the symmedian point appears as the sixth point, X(6). It lies in the open orthocentroidal disk punctured at its own center, and could be any point therein.

The symmedian point of a triangle with side lengths a, b and c has homogeneous trilinear coordinates [a : b : c].

The Gergonne point of a triangle is the same as the symmedian point of the triangle's contact triangle.

History

The French mathematician Émile Lemoine proved the existence of the symmedian point in 1873, and Ernst Wilhelm Grebe published a paper on it 1847. Simon Antoine Jean L'Huilier had also noted the point in 1809.

External links

• Symmedian and Antiparallel at cut-the-knot
• Symmedian and 2 Antiparallels at cut-the-knot
• Symmedian and the Tangents at cut-the-knot
• An interactive Java applet for the symmedian point
• Weisstein, Eric W., "Symmedian Point", MathWorld.
• Isogons and Isogonic Symmetry

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• Encyclopedia of Triangle Centers, accessed 2014-11-06.
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