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The problem:

In an acute-angled triangle $ABC$, point $H$ is the orthocenter, point $O$ is the center of the circumscribed circle, point $J$ is the center of the inscribed circle, $\angle BAC = 60°$. Prove that points $B, H, O, J, C$ lie on the same circle.*

The figure:

enter image description here


I didn't know how to even approach this problem, so I used a hint from the textbook, that says the following:

Prove that the points $H, O, J$ belong to the points locus, from which the segment $BC$ is seen from an angle of $120°$.

At first I saw that $\angle BOC = 120°$, since it's a central angle that rests on the same arc as the inscribed $\angle BAC$ of the given circle. After quite some time of looking at the figure, I also saw that $\angle BHC = 120°$, since it's simply adjacent to the angle that's measure is $60°$. I got stuck at the point where needs to be proven that $\angle BJC = 120°$ as well. I can't see how to prove that.

So I've tried to prove what's initially asked, but at that point I did stuck completely, I don't understand how the given in the textbook hint proves that they are on the same circle.
How to prove that ?

P.s. Please, note, that this problem is from an 8th grade school textbook, so any angle functions or similar, more advanced tools cannot be used here. The problem itself is on the matter "Inscribed and central angles".

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  • $\begingroup$ This might be useful: en.wikipedia.org/wiki/… $\endgroup$
    – Sathvik
    Commented Jun 14, 2023 at 15:16
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    $\begingroup$ $$\angle JBC+\angle JCB={1\over2}(\angle ABC+\angle ACB)={1\over2}\cdot120°=60°.$$ $\endgroup$ Commented Jun 14, 2023 at 15:26
  • $\begingroup$ Dicendo homines ut dicant efficere solere. $\endgroup$
    – Angelo
    Commented Jun 14, 2023 at 17:07
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    $\begingroup$ @Angelo Not common to see here a quote of Ciceron :) $\endgroup$
    – Jean Marie
    Commented Jun 14, 2023 at 17:40
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    $\begingroup$ @Intelligentipauca, oh gosh... Thank you! $\endgroup$ Commented Jun 15, 2023 at 11:34

2 Answers 2

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You have shown that $BOHC$ is a cyclic quadrilateral, since $\angle BOC=\angle BHC$.
It is easy to show that $$\angle BJC=90+\frac{\angle{A}}{{2}}=120^{\circ}$$ Therefore, $B,O,J,H,C$ all lie on the same circle.

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    $\begingroup$ This is the first result in Theorem 1 in this publication $\endgroup$
    – Jean Marie
    Commented Jun 14, 2023 at 22:22
  • $\begingroup$ Thank you for the answer, but since we know that all the angles, with the vertexes $O, J, H$ equal to $120°$, how can we imply that they are on the same circle. I don't get it. The publication that @JeanMarie shared also doesn't provide a proof for the statement. And cyclic quadrilaterals are also beyond the topic, it wasn't in the curriculum yet. $\endgroup$ Commented Jun 15, 2023 at 12:27
  • $\begingroup$ Draw the circumcircle of $\triangle BOC$, then we can conclude the rest from the first theorem in this: cuemath.com/geometry/concyclic-points $\endgroup$
    – Sathvik
    Commented Jun 16, 2023 at 5:47
  • $\begingroup$ I guess you should include this additional link to your initial answer. Thank you! $\endgroup$ Commented Jun 16, 2023 at 8:22
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J is center of inscribed circle that means it is the intersection of bisectors of angles of triangle. Now we use this theorem that says :

$\widehat {BJC}= 90 +\frac {\angle BAC}2=90+30=120^o$

So J must also be on the circle.

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  • $\begingroup$ The biggest problem was to prove that all the points are on the same circle, when all the angles at their vertexes are equal. But @Sathvik already provided a link that proves it. Thanks anyway! $\endgroup$ Commented Jun 16, 2023 at 8:24

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