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X(2), X(6), X(13), X(14), X(15), X(16), X(111), X(368), X(524), X(5000), X(5001), X(11537), X(11549), X(32618), X(32619), X(39162), X(39163), X(39164), X(39165), X(47072), X(47073) isogonal conjugate of X(368) vertices of the second and fourth Brocard triangles real foci F1, F2 of the inscribed Steiner ellipse. foci of the Kellipse (inellipse with center K when the triangle ABC is acute angle) other points below 

K018 is a circular isogonal focal nK with root X(523) and singular focus X(111). The real asymptote is the homothetic of the line GK under h(X111, 2). K018 is also the orthopivotal cubic O(X6) and Z+(L) with L = X(3)X(6) in TCCT p.241. See also Z+(O) = CL025 and CL034. K018 is spK(X524, X2) or spK(X524, X6) in CL055, see also Table 48 and Table 55. It is a member of the class CL061. See Table 52 (BrocardGrebe pencil) and another pencil in K1156. Locus properties




Let M be a variable point on the line X(2)X(6). The parallel at X(6) to the line X(111)M meets the circle X(2)X(111)M at two points M1, M2 on K018. Similarly, the parallel at X(2) to the line X(111)M meets the circle X(6)X(111)M at two points N1, N2 on K018. These four points are concyclic on (C) and (C) is orthogonal to the circle with diameter X(2)X(6). The center Ω of (C) is the intersection of the perpendicular at M to the line X(2)X(6) and the perpendicular bisector of X(111)M. Hence Ω lies on the parabola (P) with focus X(111) and directrix the line X(2)X(6). When M traverses the line X(2)X(6), (C) envelopes a circular cubic with focus X(111) passing through X(524), X(597). This cubic meets K018 only on the line at infinity, each common point being counted three times. 



K018 is invariant under isogonal conjugation with respect to ABC but also with respect to any proper triangle GPQ where G is the centroid of ABC and P, Q are two points of K018 collinear with the Lemoine point K. In particular, it is invariant under isogonal conjugation with respect to X(2)X(13)X(14) like the related K733. Other remarkable triangles are X(2)X(15)X(16) and X(2)X(6)X(111). Recall that the circumcircle of X(2)X(6)X(111) is the polar conic of the singular focus X(111). The in/excenters of any triangle GPQ lie on the axes of the Steiner ellipses which are the bisectors at G in GPQ. The roles of G and K can also be swapped and then K018 is invariant under isogonal conjugation with respect to any proper triangle KPQ where P, Q are two points of K018 collinear with the centroid G. Examples of such triangles : X(6)X(13)X(16) and X(6)X(14)X(15). The in/excenters of any triangle KPQ lie on the axes of the inconic with center K. It can be seen that two isogonal conjugates on K018 are also isogonal conjugates with respect to any of these triangles GPQ and KPQ. 



Miscellaneous properties 

The polar conic of X(110) decomposes into the Brocard axis and the trilinear polar of the root X(523). It follows that the six tangents at K, X(15), X(16) and the three points on the sidelines of ABC concur at X(110). The common tangential of X(13) and X(15) is T1 and the common tangential of X(14) and X(16) is T2. These two points are collinear with the singular focus X(111). The third points on X(13)X(15) and X(14)X(16) are N1, N2 respectively also collinear with X(111). The midpoint of N1N2 is X(230) on GK. The lines T1N2 and T2N1 are parallel to the asymptote. The tangents at X(13) and X(14) met at the reflection of X(110) about G. The polar conic of X(111) is the circle through G, O, K, X(691) and naturally X(111). The tangent at X(111) to this circle meets the asymptote at X on the cubic. Recall that this asymptote is the homothetic of the axis GK under h(X111, 2). The polar conic of the point at infinity X(524) – that of the line GK – is a rectangular hyperbola passing through X(597), having one asymptote perpendicular at X(111) to GK and the other that of the cubic. *** The bisectors of the lines X(111)X(524) and X(111)X contain X(2469), X(2470) and the centers of anallagmaty E1, E2 which obviously also lie on the previous rectangular hyperbola. The cubic is then invariant under the two inversions with pole Ei which swap X(111) and Ej. *** K018 is also invariant under the Psi transformation which is the product of the reflection about one axis of the Steiner inellipse and the inversion with circle that of diameter F1F2, the foci of the ellipse. See also "Orthocorrespondence and Orthopivotal Cubics", §5 and K022. K018 is a Psicubic as in Table 60. Psi is the involution that swaps any point of the plane to the center of its polar conic in the McCay or Kjp cubics. In particular, Psi swaps A, B, C and the vertices of the second Brocard triangle. Hence Psi transforms the sidelines of ABC into the McCay circles. The analogous involution Psi2 introduced at K1142 also keeps K108 unchanged. Here the McCay and Kjp cubics are replaced by the stelloid K598 with radial center X(6). *** K018 is also invariant under the KW involution described in the page Table 62. See the related K185. *** K018 contains the equiBrocard center X(368), its isogonal conjugate X(368)* and its Psi image. Recall that X(368) also lies on the Wallace hyperbola (the anticomplement of the Kiepert hyperbola) and on the three equibrocardian focal cubics K083. 

The polar conic of G in K018 is a rectangular hyperbola with center K that passes through X(2), X(4), X(194), X(1689), X(1690), X(1992), X(3413), X(3414) hence it is homothetic to (K). It also contains the foci of the orthic inconic hence the tangents at these four points concur at G. Its equation is : ∑(b^2  c^2)(SA x^2 + a^2 y z) = 0. Its Psiimage is the strophoid K1274. More generally, the polar conic of every point on the line GK is a rectangular hyperbola since GK is the orthic line of the cubic. That of K is remarkable with equation ∑(b^2  c^2)(b^2 c^2 x^2 + 4 a^2 SA y z) = 0. Its center is G and it passes through X(3), X(6), X(381), X(599), X(2574), X(2575), hence it is homothetic to the Jerabek hyperbola. It also contains the foci of the Steiner inellipse hence the tangents at these four points concur at K. See also the strophoid K1275 for similar properties. 
