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equal segments on radiuses
danepale   8
N 13 minutes ago by zuat.e
Source: Croatia TST 2016
Let $ABC$ be an acute triangle with circumcenter $O$. Points $E$ and $F$ are chosen on segments $OB$ and $OC$ such that $BE = OF$. If $M$ is the midpoint of the arc $EOA$ and $N$ is the midpoint of the arc $AOF$, prove that $\sphericalangle ENO + \sphericalangle OMF = 2 \sphericalangle BAC$.
8 replies
danepale
Apr 25, 2016
zuat.e
13 minutes ago
2- player game on a strip of n squares with two game pieces
parmenides51   2
N 33 minutes ago by Gggvds1
Source: 2023 Austrian Mathematical Olympiad, Junior Regional Competition , Problem 3
Alice and Bob play a game on a strip of $n \ge  3$ squares with two game pieces. At the beginning, Alice’s piece is on the first square while Bob’s piece is on the last square. The figure shows the starting position for a strip of $ n = 7$ squares.
IMAGE
The players alternate. In each move, they advance their own game piece by one or two squares in the direction of the opponent’s piece. The piece has to land on an empty square without jumping over the opponent’s piece. Alice makes the first move with her own piece. If a player cannot move, they lose.

For which $n$ can Bob ensure a win no matter how Alice plays?
For which $n$ can Alice ensure a win no matter how Bob plays?

(Karl Czakler)
2 replies
parmenides51
Mar 26, 2024
Gggvds1
33 minutes ago
Incenters and Circles
rkm0959   6
N 38 minutes ago by happypi31415
Source: Korean National Junior Olympiad Problem 1
In a triangle $\triangle ABC$ with incenter $I$,
Let $D$ = $AI$ $\cap$ $BC$
$E$ = incenter of $\triangle ABD$
$F$ = incenter of $\triangle ACD$
$P$ = intersection of $\odot BCE$ and $\overline {ED}$
$Q$ = intersection of $\odot BCF$ and $\overline {FD}$
$M$ = midpoint of $\overline {BC}$

Prove that $D, M, P, Q$ concycle
6 replies
rkm0959
Nov 2, 2014
happypi31415
38 minutes ago
Reflected point lies on radical axis
Mahdi_Mashayekhi   6
N 38 minutes ago by khanhnx
Source: Iran 2025 second round P4
Given is an acute and scalene triangle $ABC$ with circumcenter $O$. $BO$ and $CO$ intersect the altitude from $A$ to $BC$ at points $P$ and $Q$ respectively. $X$ is the circumcenter of triangle $OPQ$ and $O'$ is the reflection of $O$ over $BC$. $Y$ is the second intersection of circumcircles of triangles $BXP$ and $CXQ$. Show that $X,Y,O'$ are collinear.
6 replies
Mahdi_Mashayekhi
Apr 19, 2025
khanhnx
38 minutes ago
Gcd of N and its coprime pair sum
EeEeRUT   19
N 41 minutes ago by HamstPan38825
Source: EGMO 2025 P1
For a positive integer $N$, let $c_1 < c_2 < \cdots < c_m$ be all positive integers smaller than $N$ that are coprime to $N$. Find all $N \geqslant 3$ such that $$\gcd( N, c_i + c_{i+1}) \neq 1$$for all $1 \leqslant i \leqslant m-1$

Here $\gcd(a, b)$ is the largest positive integer that divides both $a$ and $b$. Integers $a$ and $b$ are coprime if $\gcd(a, b) = 1$.

Proposed by Paulius Aleknavičius, Lithuania
19 replies
EeEeRUT
Apr 16, 2025
HamstPan38825
41 minutes ago
Question on Balkan SL
Fmimch   4
N an hour ago by BreezeCrowd
Does anyone know where to find the Balkan MO Shortlist 2024? If you have the file, could you send in this thread? Thank you!
4 replies
Fmimch
Apr 30, 2025
BreezeCrowd
an hour ago
(3^{p-1} - 1)/p is a perfect square for prime p
parmenides51   4
N an hour ago by Rayvhs
Source: 2017 Saudi Arabia JBMO TST 1.2
Find all prime numbers $p$ such that $\frac{3^{p-1} - 1}{p}$ is a perfect square.
4 replies
parmenides51
May 28, 2020
Rayvhs
an hour ago
Rays, incircle, angles...
mathisreal   3
N an hour ago by Assassino9931
Source: Rioplatense L-3 2022 #4
Let $ABC$ be a triangle with incenter $I$. Let $D$ be the point of intersection between the incircle and the side $BC$, the points $P$ and $Q$ are in the rays $IB$ and $IC$, respectively, such that $\angle IAP=\angle CAD$ and $\angle IAQ=\angle BAD$. Prove that $AP=AQ$.
3 replies
mathisreal
Dec 13, 2022
Assassino9931
an hour ago
Find the value
sqing   0
2 hours ago
Source: Own
Let $ a,b $ be real numbers such that $ (a^2 + b^2) (a + 1) (b + 1) =  a ^ 3 + b ^ 3 =2 $. Find the value of $ a b .$

Let $ a,b $ be real numbers such that $ (a^2 + b^2) (a + 1) (b + 1) = 2 $ and $ a ^ 3 + b ^ 3 = 1 $. Find the value of $ a + b .$
0 replies
sqing
2 hours ago
0 replies
Wordy Geometry in Taiwan TST
ckliao914   9
N 2 hours ago by Scilyse
Source: 2023 Taiwan TST Round 3 Mock Exam 6
Given triangle $ABC$ with $A$-excenter $I_A$, the foot of the perpendicular from $I_A$ to $BC$ is $D$. Let the midpoint of segment $I_AD$ be $M$, $T$ lies on arc $BC$(not containing $A$) satisfying $\angle BAT=\angle DAC$, $I_AT$ intersects the circumcircle of $ABC$ at $S\neq T$. If $SM$ and $BC$ intersect at $X$, the perpendicular bisector of $AD$ intersects $AC,AB$ at $Y,Z$ respectively, prove that $AX,BY,CZ$ are concurrent.
9 replies
ckliao914
Apr 29, 2023
Scilyse
2 hours ago
Factorial Divisibility
Aryan-23   47
N 2 hours ago by ezpotd
Source: IMO SL 2022 N2
Find all positive integers $n>2$ such that
$$ n! \mid \prod_{ p<q\le n, p,q \, \text{primes}} (p+q)$$
47 replies
Aryan-23
Jul 9, 2023
ezpotd
2 hours ago
easy geo
ErTeeEs06   6
N May 28, 2025 by lksb
Source: BxMO 2025 P3
Let $ABC$ be a triangle with incentre $I$ and circumcircle $\Omega$. Let $D, E, F$ be the midpoints of the arcs $\stackrel{\frown}{BC}, \stackrel{\frown}{CA}, \stackrel{\frown}{AB}$ of $\Omega$ not containing $A, B, C$ respectively. Let $D'$ be the point of $\Omega$ diametrically opposite to $D$. Show that $I, D'$ and the midpoint $M$ of $EF$ lie on a line.
6 replies
ErTeeEs06
Apr 26, 2025
lksb
May 28, 2025
easy geo
G H J
G H BBookmark kLocked kLocked NReply
Source: BxMO 2025 P3
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ErTeeEs06
69 posts
#1 • 4 Y
Y by Funcshun840, Kizaruno, Rounak_iitr, GA34-261
Let $ABC$ be a triangle with incentre $I$ and circumcircle $\Omega$. Let $D, E, F$ be the midpoints of the arcs $\stackrel{\frown}{BC}, \stackrel{\frown}{CA}, \stackrel{\frown}{AB}$ of $\Omega$ not containing $A, B, C$ respectively. Let $D'$ be the point of $\Omega$ diametrically opposite to $D$. Show that $I, D'$ and the midpoint $M$ of $EF$ lie on a line.
This post has been edited 1 time. Last edited by ErTeeEs06, Apr 26, 2025, 11:15 AM
Reason: latex
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wassupevery1
325 posts
#2 • 4 Y
Y by Funcshun840, Kizaruno, NicoN9, GA34-261
bruh

Solution
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mikimoto12
877 posts
#3 • 2 Y
Y by Kizaruno, GA34-261
first thing i thought of
ugly
yeah in hindsight the other sol is much easier and pretty obvious
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NicoN9
164 posts
#4 • 1 Y
Y by GA34-261
I won't write a full solution, but here is my solution outline:

Let $ABC$ be a unitcircle. There exists $u, v, w$, such that $a=u^2$, $b=v^2$, $c=w^2$. And we have $d=-vw$, $e=-wu$, $f=-uv$, Further, $m=-\frac{1}{2}(v+w)u$, $i=-(uv+vw+wu)$, $d'=-vw$. Just see that these are collinear, which is not so hard.
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InterLoop
279 posts
#5 • 1 Y
Y by GA34-261
solution
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Adywastaken
74 posts
#6 • 1 Y
Y by GA34-261
Just notice $a=x^2$, $b=y^2$, $c=z^2$, $d=-yz$, $e=-zx$, $f=-xy$, $j=-(xy+yz+zx)$, $d'=yz$, $m=\frac{-xy-yz}{2}$
Then,
\[
\frac{2yz+xy+xz}{\frac{2yz+xy+xz}{2}}=2 \in \mathbb{R}
\]
This post has been edited 1 time. Last edited by Adywastaken, May 20, 2025, 3:33 PM
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lksb
183 posts
#7 • 1 Y
Y by GA34-261
Pretty straightforward complex bash
Let $(ABC)$ be the unit circle
$a=x^2, b=y^2, c=z^2, i=-xy-xz-yz, d=-yz, e=-xz, f=-xy, d'=yz, m=-\frac{xy+xz}{2}$
$$I-D'-M\iff \frac{d'-i}{d'-m}=\frac{yz+xy+xz+yz}{yz+\frac{xy+xz}{2}}=2\in\mathbb{R}$$
This post has been edited 2 times. Last edited by lksb, May 28, 2025, 2:04 AM
Reason: simplifying
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