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In a triangle $\Delta ABC$,suppose E is the midpoint of AC,F is the midpoint of AB,$BE\bot CF$,point O,I,H are the circumcenter, incenter, and orthocenter of this triangle, respectively.

"circle I" denotes the inscribed circle of the triangle ABC.

Prove: If point H is on the "circle I",then point O is on the "circle I" too.

I tried to use the knowledge of Euler's circle to solve this question,and I connect line segment OE,and I suppose $BE \bigcap CF=G$,where G is the center of gravity of $\Delta ABC$,but I don't know what to do next. Can anyone provide an answer?

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    $\begingroup$ I am supposing the "outer center" is the circumcenter, the "inner center" is the incenter, and the "vertical center" is the orthocenter. $\endgroup$ Commented Nov 24, 2024 at 10:53
  • $\begingroup$ If $H,I$ are points, what does it mean for $H$ to be on circle $I$ ? $\endgroup$ Commented Nov 24, 2024 at 11:00
  • $\begingroup$ Thank you for your correction! $\endgroup$ Commented Nov 24, 2024 at 11:00
  • $\begingroup$ H is on the inscribed circle of the triangle ABC. $\endgroup$ Commented Nov 24, 2024 at 11:02
  • $\begingroup$ $𝐵E\cap 𝐶𝐹=G$ follows from the similitude between $ABC$ and $AFE$, and from the similitude between the two triangles formed by $BE$, $CF$ with $FE$, $AB$. $\endgroup$ Commented Nov 24, 2024 at 11:47

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COMMENT.-There are many triangles that satisfy the initial conditions and not all of them verify the possibility that $H$ is in the incircle. The cases in which this is true, it seems to me that are very few and even perhaps just one (I mean all the answers could be similar).

Anyway, all these kind of triangles $\triangle{ABC}$ can be defined by the points $B=(2a,0),(-a,0), C=(0,2b),(0,-b)$ which determines the point $A=(-2a,-2b)$. From this, the circumcenter can be deduced from the system $$\begin{cases}b(y+b)=-2ax\\2by=-a(x+a)\end{cases}\implies (x_O,y_O)=\left(\frac{a^2-2b^2}{3a},\frac{b^2-2a^2}{3b}\right)$$ Similarly one has for the orthocenter $$(x_H,y_H)=\left(\frac{4b^2-2a^2}{3a},\frac{4a^2-2b^2}{3b}\right)$$ On the other hand the coordinates of the incenter are given by $$x_I=\frac{2a(\sqrt{a^2+4b^2}-\sqrt{a^2+b^2})}{\sqrt{a^2+b^2}+\sqrt{4a^2+b^2}+\sqrt{a^2+4b^2}}\\y_I=\frac{2b(\sqrt{4a^2+b^2}-\sqrt{a^2+b^2})}{\sqrt{a^2+b^2}+\sqrt{4a^2+b^2}+\sqrt{a^2+4b^2}}$$ With the side $\overline{AC}$ whose line have equation $2bx-ay+2ab=0$, we have for the inradius $r$ $$r=\frac{|2bx_I-ay_I+2ab|}{\sqrt{a^2+b^2}+\sqrt{4a^2+b^2}+\sqrt{a^2+4b^2}}$$ Parameters $a,b$ giving suitable triangles can be taken from the equalities $$\overline{IH}=\overline{IO}=r$$ Calculations become less arduous if we make $a=1$ (so for each value of the resultant $b$, all similar triangle is available).

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enter image description here

In picture N is the center of nine point circle and is the mid point of OH. Now all we have to show is that $ON\bot BP$ Wich Will prove that O is also on in circle.The data that $FC\bot BE$ deduces that :

$arc BC+arc QS=180^o$

Now the rotation of construction $QC\bot BS$ about vertex B such that :

1- S coincides on P

2- Q coincides on U

must result in the coincidence of C on M which in turn results in $UM\bot BP$ which deduces that the diameter UM, which HO locates on bisects the chord BP. In this way BP is perpendicular bisector of the chord HO that says O is also on the incircle.

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  • $\begingroup$ "diameter QC, which FT locates on" => Are you saying that CFTQ is the diameter of the circumcircle? If so, why doesn't it pass through O? $\endgroup$ Commented Nov 24, 2024 at 16:16
  • $\begingroup$ FWIW This makes no sense to me. Since E, F are interchangeable, saying that $T$ is the midpoint of $BS$ implies that $T$ is the midpoint of $QC$, which doesn't seem to be the case. $\quad$ Also, it would be helpful to define your points (apart from just the diagram) and explain the relationships. $\endgroup$ Commented Nov 24, 2024 at 16:24
  • $\begingroup$ @CalvinLin, You are right. Thanks for pointing it out. I will revise my answer later. Too tired now! $\endgroup$ Commented Nov 24, 2024 at 18:06
  • $\begingroup$ @sirous How do you know for sure that the center of the nine-point circle $N$ is the midpoint of $OH$? $\endgroup$ Commented Dec 6, 2024 at 16:49
  • $\begingroup$ @YNK, This is a well known theorem. You can find it in WIKI. $\endgroup$ Commented Dec 6, 2024 at 19:30

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