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Let P be a point in the plane of the reference triangle ABC and denote by
The following table gives the loci of P such that two of these triangles are orthologic. Note the frequent occurence of the McCay cubic K003 and see Table 22 for a generalization. true means the property is true for any P. In this case, the orthology center(s) are given. L denotes the line at infinity, C denotes the circumcircle. 3H is the union of three circumhyperbolas centered on the cevian lines of X(25). Each one is tangent at two vertices of ABC to the symmedians and at the last vertex to the circumcircle. For any point on 3H, cacP is degenerate into a "flat" triangle. 



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Loci related to two orthologic cevian triangles Let P be a fixed point not lying on the sidelines of the reference triangle ABC with cevian triangle PaPbPc. Recall that PaPbPc is the pedal triangle of some point Q if and only if P lies on the Lucas cubic K007 in which case Q lies on the Darboux cubic K004. Note that the line PQ always contains the de Longchamps point X(20). Let M be a variable point with cevian triangle MaMbMc. The locus of M such that the triangles PaPbPc and MaMbMc are orthologic is a circumcubic K(M). For any point M on K(M), the perpendiculars dropped from the vertices of PaPbPc on the sidelines of MaMbMc concur at O1 and the perpendiculars dropped from the vertices of MaMbMc on the sidelines of PaPbPc concur at O2. These points O1, O2 are called the centers of orthology. The loci of O1, O2 when M traverses K(M) are two other cubics K(O1), K(O2). Properties of K(M) K(M) is a circumcubic passing through : • the vertices Ga, Gb, Gc of the antimedial triangle where the tangents are concurrent • P • H÷ctP and ta(H÷P) • U, V, W on the sidelines of ABC where U also lies on the perpendicular at A0 (see below) to PbPc
Properties of K(O1) K(O1) contains : • Pa, Pb, Pc • the orthocenters H of ABC and Hp of PaPbPc • A1, B1, C1 where A1 is the intersection of the perpendiculars at Pb, Pc to AB, AC respectively • the infinite points of the altitudes of ABC (the three real asymptotes of K(O2) pass through the midpoints of PaPbPc)
Properties of K(O2) K(O2) is a circumcubic passing through : • the orthocenters H of ABC and Hp of PaPbPc • A0, B0, C0 where A0 is the intersection of the perpendiculars at B, C to PaPb, PaPc respectively • the infinite points of the altitudes of PaPbPc (the three real asymptotes of K(O1) pass through the midpoints of ABC) *** An interesting special case It is known that the cevian triangle PaPbPc of P is the pedal triangle of some point Q if and only if P lies on the Lucas cubic K007 in which case Q lies on the Darboux cubic K004. The three cubics above have the following additional properties : K(M) is pK(G, tgQ) and contains G, tP, gQ, tgQ, agQ. Note that the pivot tgQ lies on K183. K(O1) is a central cubic passing through Q, gQ with center the complement of Q with respect to the cevian triangle of P. K(O1) is a pK with respect to this latter triangle and its pivot is the crosspoint of H and gQ which is also the reflection of Q about its center. K(O2) is pK(G/cgQ, agQ), a central cubic passing through gQ, cgQ (its center), agQ. The following table gives a selection of such cubics. 



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