- The three diagonal points P, Q, R of ABCD.

- The six conjugates U

- The two (if existent) fixed points X

c is the image of the line (d) through the quadratic transformation F, defined by the bundle of conics generated by the two pairs of intersecting lines (AB,CD) and (BC,DA). As usual for this transformation, the images of lines are conics passing through the singular points of the bundle, which are P, Q, R. To see that U

That c should pass through X

Note that {A,B,C,D} are the fixed points of the quadratic transformation F.

Note also that poins X

FourPtsAndTangent.html

Quadratic_Transformation.html

Quadratic_Transformation2.html

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