3
$\begingroup$

1. Introduction

The Sawayama-Thebault theorem is one of the best nice theorem in plane geometry. The theorem has a long history. It was published in AMM in 1938 the first solution appeared in 1973 with 24 pages), You can see detail in here and here.

Theorem 1 (Sawayama-Thebault): Through the vertex $A$ of a triangle $ABC$, a straight line $AD$ is drawn, cutting the side $BC$ at $D$. $I$ is the center of the incircle of triangle $ABC$. Let $P$ be the center of the circle which touches $DC$, $DA$ at $E$, $F$, and the circumcircle of ABC, and let Q be the center of a further circle which touches $DB$, $DA$ in $G$, $H$ and the circumcircle of $ABC$. Then $P$, $I$ and $Q$ are collinear

Source of the Figure 1 in here

I have worked for four years to find a real nice generalization of the Thebault theorem and publish in here since two years ago, but has no proof. So I am looking for the proof of the generalization of the Sawayama-Thebault theorem as follows:

2. Generalization of the Sawayama-Thebault theorem

Problem 2: ( See Figure 2). Let $ABC$ be a triangle the red circle through $B$, $C$, the blue circle tangent to $AB$, $AC$ and the red circle. Let $P$ be arbitrary point in the plane, through $P$ construct two tangent lines $PD$, $PE$ to the blue circle. Let $(O_1)$ is the yellow circle tanegnt to the right tangent line $PD$, tangent to the red circle and the sideline $BC$; $(O_2)$ is the yellow circle tanegnt to the left tangent line $PF$, tangent to the red circle and tangent to the sideline $BC$. Then three points $O_1O_2$ through a fixed point when $P$ be moved on a given line.

enter image description here

3. Construct the two yellow circles?

Problem 3: (The generalization of the Sawayama Lemma) The lines $GF$, $HK$ through the incenter of $ABC$ (Figure 2)

4. Construct the point $I'$

enter image description here

See also:

$\endgroup$
  • 1
    $\begingroup$ What is the definition of the point I' ? $\endgroup$ – godelian Dec 17 '18 at 15:29
  • $\begingroup$ @godelian , Thank You very much I updated $\endgroup$ – Đào Thanh Oai Dec 17 '18 at 15:32
  • 1
    $\begingroup$ Your green circle looks blue $\endgroup$ – Fedor Petrov Dec 17 '18 at 15:33
  • $\begingroup$ Thank You dear Professor @FedorPetrov $\endgroup$ – Đào Thanh Oai Dec 17 '18 at 15:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.