QA-Tf9: QA-5th Point Tangent

QA-Tf9 maps a point P to the tangent at P to the circumscribed conic of P1,P2,P3,P4,P. It is the dual of QL-Tf7.

Construction

Definition of Triangle Transformation TR-Tfx:

TR-Tfx(ABC,P,Pi) = P-Isoconjugate wrt triangle ABC of P.

It can be constructed as QA-Tf2(Pi) wrt quadrangle (Pa,Pb,Pc,P) , where Pa,Pb,Pc are the vertices of the Anticevian Triangle of P wrt ABC (construction of Eckart Schmidt, see [34], QFG#2145, #2148).

Let P1, P2, P3, P4 be the basic points of the Reference Quadrangle, also referred to as Pi (i=1,2,3,4). Let P be some random point. Let QA-DT = Diagonal Triangle QA-Tr1.

Then QA-Tf9(P) is the line on which P and the 4 occurrences of Pi’ = TR-Tfx(QA-DT,P,Pi) are collinear.

Construction of 5^th Point Tangent at an infinity point:

The 5^th Point Tangent at an infinity point is actually the infinity point of the asymptote of a circumscribed QA-Hyperbola.

Let P0 be the Infinity Point of some line named L0.

Let L1 and L2 be two random lines.

Then 5th-point-tangent QA-Tf9(P0) can be drawn by constructing QA-Tf18(L1,L0) and QA-Tf18(L2,L0) and by connecting both obtained QA-Tf14-points. See [34], QFG#3713.

The envelope of all 5th-point-tangents at an infinity point is a quartic touching all six sides of the Quadrangle. It is the same quartic as mentioned by Eckart Schmidt in [34], QFG#141.

CT-Coordinates

Let P (x : y : z) be the 5^th point, then QA-Tf9(P) is

(p y z (r y – q z) : q x z (-r x + p z) : r x y (q x – p y))

Properties

• QA-Tf9(QA-Px) is the line through QA-Px, QA-Tf2(QA-Px), QA-Tf5(QA-Px).
• When QA-Px and QA-Py are involutary conjgates (QA-Tf2), then QA-Tf9(QA-Px)=QA-Tf9(QA-Py)=QA-Px.QA-Py.
• The Crosspoint(P,Pi) wrt triangle Pj.Pk.Pl (i,j,k,l are different numbers from (1,2,3,4)) lies on the 5^th point tangent of P. See [13], keyword Crosspoint.

Table of 5^th Point Tangents

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