QA-Tf6 Quang Duong’s Transformation

This Transformation was introduced by Ngo Quang Duong. See Ref-34, QFG#1176.
Let ABCD be the Reference Quadrangle and r a random line.
Line r intersects AB, CD, AC, DB, AD, BC at MAB, MCD, MAC, MDB, MAD, MBC.
HAB, HCD, HAC, HDB, HAD, HBC is orthogonal projections of MCD, MAB, MDB, MAC, MBC, MAD on  AB, CD, AC, DB, AD, BC.
Then HAB HCD, HAC HDB, HAD HBC are concurrent in a point R being the Quang Duong’s Transformation of line r wrt Reference Quadrangle ABCD.
When vertices A,B,C,D are concyclic, then the lines HAB HCD, HAC HDB, HAD HBC will be parallel and consequently R will be an infinity point.
QA-Tf6-Quang Duongs Transformation-00
Alternative constructions (see Ref-34, Benedetto Scimemi, QFG # 1185):
  1. QA-Tf6(r) = QA-Tf3 (reflection of QA-P4 in r)
  2. QA-Tf6(r) = reflection of QA-P2 in QA-Tf3(r)
Following tables list the relations between several Quadrangle Objects.
General Behavior of the transformation. See Ref-34, ES#1187,1191.
Transformed Item
Pencil of parallel lines
Line through QA-P2
Pencil of lines perpendicular to some line L1
Pencil of lines perpendicular to line L2
Line through QA-P2 parallel to L1
Line pencil through some point P
Ci=Circle through QA-P2
Line pencil through QA-Tf6(P) perpendicular to r
Envelope=Deltoid circumscribing Ci
Line pencil through QA-Tf6(P) parallel to r
Envelope=Deltoid circumscribing Ci, homothetic with the deltoid for the Simson lies of the Diagonal Triangle (wrt QA-P12 and factor 2)
Transformation of classes of QA-objects. See Ref-34, ES#1178,1191
Transformed Item
Line through QA-P4
Line pencil through QA-P1
Circle through QA-P2 centered in midpoint QA-P1.QA-P23
Line pencil through QA-P2
Circle through QA-P2 centered in QA-P23
Line pencil through QA-P12
Circumcircle of Diagonal triangle (QA-Ci1)
Pencil of lines perpendicular to QA-L4
Pencil of lines perpendicular to QA-L1
Line through QA-P2 parallel to QA-L4
Pencil of lines perpendicular to QA-L2
Pencil of lines perpendicular to QA-P2.QA-P23
Tangents to circles (Ci1) round QA-P4
Circles (Ci2) round QA-P2
Intersection of circles Ci1 and Ci2
Circle through QA-P3
centered in QA-P2.QA-P4 ^ QA-P3.QA-P32
Radical Axes of Ci1 and Ci2
Tangents to Parabolas with focus QA-P4
Perpendicular bisector of QA-P2 and the QA-Tf6-image of the parabola directrix
Tangents to a Conic with focus QA-P4
Circle centered in the QA-Tf6-image of the secondary axis of the conic
Transformation of specific QA-Objects. See Ref-34, #1181(CvT), #1191(ES).
Transformed Item
QA-L1 (line through QA-P1,QA-P2,QA-P3)
Point on QA-P3.QA-P4
QA-L2 (line through QA-P2,QA-P4,QA-P6)
QA-L4 (line through QA-P1,QA-P6,QA-P23)
Point on line through QA-P23 parallel to QA-L1
QA-L5 (Eulerline of Diagonal Triangle)
Point on circumcircle Diagonal Triangle
QA-L9 (perpendicular bisector QA-P2.QA-P4)
Line through QA-P3 perpendicular to QA-L4
1st CT-coordinate for line (x:y:z):
- a2 (a2 p q - b2 p q + c2 p q + a2 p r + b2 p r - c2 p r + 2 a2 q r) x2
- b2 (a2 - b2 + c2) p q y2
- c2 (a2 + b2 - c2) p  r z2
- (a2 c2 q + b2 c2 q - c4 q + a2 b2 r - b4 r + b2 c2 r) p y z
+ (a4 p q - b4 p q + 2 b2 c2 p q - c4 p q + 2 a2 b2 p r + a4 q r + a2 b2 q r - a2 c2 q r) x y
+ (2 a2 c2 p q + a4 p r - b4 p r + 2 b2 c2 p r - c4 p r + a4 q r - a2 b2 q r + a2 c2 q r) x z
  • QA-Tf6(d) lies on the Orthopolar Line of r (See Ref-34, QFG#1186). For definition Orthopolar Line see Ref-13.


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