QA-Cu6: QA-P1-Involution Center Cubic

This cubic is the locus of the involution centers of all lines passing through QA-P1 (Quadrangle-Centroid). So it is the locus of Midpoints of the Involutary Conjugates on the cubic QA-Cu5.
The involution is created by the intersection points of the QA-P1-line with the sides of Quadrigon P1.P2.P3.P4 (see chapter QA-Tf1: QA-Line Involution)
QA-P22, the Reflection of the Midpoint (QA-P1,QA-P5) in QA-P1 also lies on the cubic.
Similar cubics can be constructed by taking involution centers of lines passing through another point than QA-P1. The other 2 QA-quadrigons deliver the same cubic. This makes the cubic a real QA-Cubic.
Equation CT-notation:
             r (p+q+2r) ((3q+p)x(3p+q) y) x y
            + q (p+2q+r) ((3p+r) z(3r+p) x) z x
            + p (2p+q+r) ((3r+q) y(3q+r) z) y z - (p-q)(q-r)(r-p) x y z = 0
Equation DT-notation:
q2 r2 (q2 - r2) x3 + q2 r2 (2 p2 (y - z) - (q2 - r2) (y + z)) x2
+ p2 r2 (r2 - p2) y3 + p2 r2 (2 q2 (z - x) - (r2 - p2) (x + z)) y2
+ p2 q2 (p2 - q2) z3 + p2 q2 (2 r2 (x - y) - (p2 - q2) (x + y)) z2 = 0
  • The vertices of the Quadrangle and the Midpoints of the Diagonal triangle and QA-P1, QA-P22 lie on this cubic.
  • The intersection point QA-P1.QA-P6 ^ QA-P22.QA-P29 lies on this cubic.
  • The intersection points of the lines through QA-P1 parallel to the QA-sidelines with the opposite sidelines all lie on this cubic.

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