- Pusat kongruensi Yff
- Yff center of congruence
- Triangle center
- List of triangle topics
- Congruent isoscelizers point
- Modern triangle geometry
yff center of congruence
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In geometry, the Yff center of congruence is a special point associated with a triangle. This special point is a triangle center and Peter Yff initiated the study of this triangle center in 1987.
Isoscelizer
An isoscelizer of an angle A in a triangle △ABC is a line through points P1, Q1, where P1 lies on AB and Q1 on AC, such that the triangle △AP1Q1 is an isosceles triangle. An isoscelizer of angle A is a line perpendicular to the bisector of angle A. Isoscelizers were invented by Peter Yff in 1963.
Yff central triangle
Let △ABC be any triangle. Let P1Q1 be an isoscelizer of angle A, P2Q2 be an isoscelizer of angle B, and P3Q3 be an isoscelizer of angle C. Let △A'B'C' be the triangle formed by the three isoscelizers. The four triangles △A'P2Q3, △Q1B'P3, △P1Q2C', and △A'B'C' are always similar.
There is a unique set of three isoscelizers P1Q1, P2Q2, P3Q3 such that the four triangles △A'P2Q3, △Q1B'P3, △P1Q2C', and △A'B'C' are congruent. In this special case △A'B'C' formed by the three isoscelizers is called the Yff central triangle of △ABC.
The circumcircle of the Yff central triangle is called the Yff central circle of the triangle.
Yff center of congruence
Let △ABC be any triangle. Let P1Q1, P2Q2, P3Q3 be the isoscelizers of the angles A, B, C such that the triangle △A'B'C' formed by them is the Yff central triangle of △ABC. The three isoscelizers P1Q1, P2Q2, P3Q3 are continuously parallel-shifted such that the three triangles △A'P2Q3, △Q1B'P3, △P1Q2C' are always congruent to each other until △A'B'C' formed by the intersections of the isoscelizers reduces to a point. The point to which △A'B'C' reduces to is called the Yff center of congruence of △ABC.
Properties
The trilinear coordinates of the Yff center of congruence are
sec
A
2
:
sec
B
2
:
sec
C
2
{\displaystyle \sec {\frac {A}{2}}:\sec {\frac {B}{2}}:\sec {\frac {C}{2}}}
Any triangle △ABC is the triangle formed by the lines which are externally tangent to the three excircles of the Yff central triangle of △ABC.
Let I be the incenter of △ABC. Let D be the point on side BC such that ∠BID = ∠DIC, E a point on side CA such that ∠CIE = ∠EIA, and F a point on side AB such that ∠AIF = ∠FIB. Then the lines AD, BE, CF are concurrent at the Yff center of congruence. This fact gives a geometrical construction for locating the Yff center of congruence.
A computer assisted search of the properties of the Yff central triangle has generated several interesting results relating to properties of the Yff central triangle.
Generalization
The geometrical construction for locating the Yff center of congruence has an interesting generalization. The generalisation begins with an arbitrary point P in the plane of a triangle △ABC. Then points D, E, F are taken on the sides BC, CA, AB such that
∠
B
P
D
=
∠
D
P
C
,
∠
C
P
E
=
∠
E
P
A
,
∠
A
P
F
=
∠
F
P
B
.
{\displaystyle \angle BPD=\angle DPC,\quad \angle CPE=\angle EPA,\quad \angle APF=\angle FPB.}
The generalization asserts that the lines AD, BE, CF are concurrent.
See also
Congruent isoscelizers point
Central triangle