In the Encyclopedia of Triangle Centers the symmedian point appears as the sixth point, X(6).[2] It lies in the open orthocentroidal disk punctured at its own center, and could be any point therein.[3]
The symmedian point of a triangle with side lengths a, b and c has homogeneous trilinear coordinates [a : b : c].[2]
The Gergonne point of a triangle is the same as the symmedian point of the triangle's contact triangle.[4]
[0] The symmedian at C, of a triangle ABC, is defined as the symmetric line of the median from C, with respect to the bisector of C (see Symmedian_0.html for its properties).
[1] The three symmedians of ABC pass through the Symmedian point K of the triangle.
[2] K coincides with the Gergonne point of the tangential triangle A'B'C' of ABC.
[3] Thus, the two triangles ABC and A'B'C' are point-perspective and by Desargues (see Desargues.html ), they are also line-perspective. The corresponding line A*B* is the Trilinear polar of the triangle ABC with respect ot K and is called the Lemoine-Axis of the triangle.
[4] A* is the intersection point of BC and B'C', B* is the intersection point of CA and C'A' etc.
[5] A* is the pole of AA' with respect to the circumcircle of ABC, B* is the pole of BB' etc.
[6] By the duality of the polarity, this implies that K is the pole of the trilinear polar. Hence OK is orthogonal to the trilinear polar.
[7] Line (OK) is the Brocard-Axis of the triangle.
[8] C* is the center of the Apollonian circle of ABC passing through C. Analogous properties hold for B* and A*. Thus the three Apollonian circles have their centers on the Lemoine axis.