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Euclidean geometry is a mathematical system attributed to the Alexandrian Greek mathematician Euclid, which he described in his textbook on geometry: the Elements. Euclid's method consists in assuming a small set of intuitively appealing axioms, and deducing many other propositions (theorems) from these.

2 votes

Dao's theorem on six circumcenters associated with a cyclic hexagon

The solution of problem in our paper On an Extension of Miquel's Theorem to a Cyclic Hexagon; Relative configuration in here On the eight circles theorem and its dual
Đào Thanh Oai's user avatar
-1 votes
2 answers
318 views

A Erdős–Mordell Like inequality

Ono's inequality is true for acute triangle but false with general triangles. The inequality as follows is false with general triangls but I think it true with acute triangle (follows answer by Fedor …
Đào Thanh Oai's user avatar
3 votes
0 answers
298 views

A problem on configuration of Dao's theorem on six circumcenters

Abstract: In the figure belows: Three lines through center of pair opposite red circle are concurrent. This is a statement of Dao's theorem on six circumcenter, a new theorem in plane geometry which w …
Đào Thanh Oai's user avatar
2 votes
0 answers
211 views

A generalization of the Archimedean circle

I proposed a generalization of the Archimedean circle : In this figure $M$ is the midpoint of $AB$, $DE$; $(G)$, $(H)$, $(M)$ are the semicircles. Then two yellow circles are congruent. Question: Is t …
Đào Thanh Oai's user avatar
1 vote
0 answers
111 views

Is the formula known? and can we generalized for higher dimensions?

In this configuration as follows, we have a nice formula: $$\cos(\varphi)=\frac{OF.OE+OB.OC}{OF.OB+OE.OC}$$ Is the formula known? and can we generalized for higher dimensions?
Đào Thanh Oai's user avatar
3 votes
1 answer
403 views

Generalization of Tucker circle, Conway circle and van Lamoen circle

Theorem 9.1 in this paper as follows is a generalization of Turker circle. Turker circles is a generalization of many circles as: Cosine Circle, circum circle, First Lemoine Circle, Gallatly Circle, K …
Đào Thanh Oai's user avatar
1 vote

An new equilateral triangle related to the Morley triangle

Today I have just been found that the theorem above was found early by Dr. Floor van Lamoen. His paper Equilateral chordal triangles.
Đào Thanh Oai's user avatar
11 votes

Does this geometry theorem have a name?

This is theorem 2 (the Parallel tangent theorem) in "Two Applications of the Generalized Ptolemy Theorem" by Shay Gueron.
Đào Thanh Oai's user avatar
3 votes
1 answer
173 views

Relative to Isoperimetric inequality with n-polygon

Since Brahmagupta's formula and Bretschneider's formula we have the inequality: Any two quardrilaterals $A_1A_2A_3A_4$ and $B_1B_2B_3B_4$ with the same sidelengths and $A_1A_2A_3A_4$ is a cyclic b …
Đào Thanh Oai's user avatar
2 votes
1 answer
371 views

Yiu's equilateral triangle-triplet points

In more than 2300 years since Euclid's Elements appear, there were only two equilateral triangles become famous: The Morely equilateral triangle and the Napoleon equilateral triangle. In more than 200 …
Đào Thanh Oai's user avatar
2 votes
0 answers
166 views

Pascal theorem for three dimensions

A year ago I found the Pascal theorem for three dimentions as follows: Let $(C_1)$, $(C_2)$ be two conics on the same Ellipsoid, (or Hyperboloid, or Paraboloid). Let $A_1$, $A_2$, $A_3$, $A_4$, $A_5$ …
Đào Thanh Oai's user avatar
5 votes
0 answers
341 views

$N$-$th$ closed chain of six circles

Since 2013, I found a very nice configuration: $N$-th closed chain of six circles. This is a generalization of theorem 1, problem 2 in here and theorem 2 in here and here (and is also generalizatio …
Đào Thanh Oai's user avatar
3 votes
0 answers
886 views

A generalization of the Sawayama-Thebault theorem

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 wi …
Đào Thanh Oai's user avatar
3 votes
0 answers
231 views

Are these points known? [closed]

Let $ABC$ be a triangle and $P$ be a point on the plane, $PA$, $PB$, $PC$ meet $BC$, $CA$, $AB$ at $A'$, $B'$, $C'$ respectively. From my construction by GeoGebra, I found two special points as follow …
Đào Thanh Oai's user avatar
3 votes
1 answer
503 views

An new equilateral triangle related to the Morley triangle

Morley equilateral triangle is the nice theorem in Eulidean Geometry. I found an equilateral triangle and a group circle related to the Morley triangle and angle trisectors: Let $ABC$ be a triangle …
Đào Thanh Oai's user avatar

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