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one cyclic formed by two cyclic
CrazyInMath   22
N 4 minutes ago by ItzsleepyXD
Source: EGMO 2025/3
Let $ABC$ be an acute triangle. Points $B, D, E$, and $C$ lie on a line in this order and satisfy $BD = DE = EC$. Let $M$ and $N$ be the midpoints of $AD$ and $AE$, respectively. Suppose triangle $ADE$ is acute, and let $H$ be its orthocentre. Points $P$ and $Q$ lie on lines $BM$ and $CN$, respectively, such that $D, H, M,$ and $P$ are concyclic and pairwise different, and $E, H, N,$ and $Q$ are concyclic and pairwise different. Prove that $P, Q, N,$ and $M$ are concyclic.
22 replies
+1 w
CrazyInMath
Yesterday at 12:38 PM
ItzsleepyXD
4 minutes ago
Projection of vertex onto bisectors
randomusername   8
N 6 minutes ago by AshAuktober
Source: ITAMO 2016, Problem 1
Let $ABC$ be a triangle, and let $D$ and $E$ be the orthogonal projections of $A$ onto the internal bisectors from $B$ and $C$. Prove that $DE$ is parallel to $BC$.
8 replies
randomusername
May 11, 2016
AshAuktober
6 minutes ago
Determining Integers From Sums
oVlad   1
N 26 minutes ago by removablesingularity
Source: Romania Junior TST 2025 Day 1 P3
Let $n\geqslant 3$ be a positiv integer. Ana chooses the positive integers $a_1,a_2,\ldots,a_n$ and for any non-empty subset $A\subseteq\{1,2,\ldots,n\}$ she computes the sum \[s_A=\sum_{k
\in A}a_k.\]She orders these sums $s_1\leqslant s_2\leqslant\cdots\leqslant s_{2^n-1}.$ Prove that there exists a subset $B\subseteq\{1,2,\ldots,2^n-1\}$ with $2^{n-2}+1$ elements such that, regardless of the integers $a_1,a_2,\ldots,a_n$ chosen by Ana, these can be determined by only knowing the sums $s_i$ with $i\in B.$
1 reply
1 viewing
oVlad
Saturday at 9:45 AM
removablesingularity
26 minutes ago
Colouring numbers
kitun   4
N 2 hours ago by quasar_lord
What is the least number required to colour the integers $1, 2,.....,2^{n}-1$ such that for any set of consecutive integers taken from the given set of integers, there will always be a colour colouring exactly one of them? That is, for all integers $i, j$ such that $1<=i<=j<=2^{n}-1$, there will be a colour coloring exactly one integer from the set $i, i+1,.... , j-1, j$.
4 replies
kitun
Nov 15, 2021
quasar_lord
2 hours ago
Arithmetic mean of all values
Zelderis   1
N 2 hours ago by Rohit-2006
Source: Brazil Undergrad MO - Galois-Noether 2018 #14
What is the arithmetic mean of all values of the expression $ | a_1-a_2 | + | a_3-a_4 | $
Where $ a_1, a_2, a_3, a_4 $ is a permutation of the elements of the set {$ 1,2,3,4 $}?
1 reply
Zelderis
Nov 26, 2019
Rohit-2006
2 hours ago
If p^23, p^24, q^23, q^24 are in AP, then it also includes p and q
Tintarn   4
N 3 hours ago by de-Kirschbaum
Source: All-Russian MO 2024 10.1
Let $p$ and $q$ be different prime numbers. We are given an infinite decreasing arithmetic progression in which each of the numbers $p^{23}, p^{24}, q^{23}$ and $q^{24}$ occurs. Show that the numbers $p$ and $q$ also occur in this progression.
Proposed by A. Kuznetsov
4 replies
Tintarn
Apr 22, 2024
de-Kirschbaum
3 hours ago
Trigonometric Equation
VitaPretor   0
3 hours ago
\[
\text{Given that } 0 < \theta < 90^\circ,\ \text{solve the equation: } \sin(\theta - 60^\circ)\sin\theta + \sin(54^\circ - \theta)\sin 54^\circ = 0
\]\[
\text{What is the value of } \theta\ (\text{in degrees})\ \text{that satisfies the equation?}
\]
0 replies
VitaPretor
3 hours ago
0 replies
AO and KI meet on $\Gamma$
Kayak   28
N 3 hours ago by Ilikeminecraft
Source: Indian TST 3 P2
Let $ABC$ be an acute-angled scalene triangle with circumcircle $\Gamma$ and circumcenter $O$. Suppose $AB < AC$. Let $H$ be the orthocenter and $I$ be the incenter of triangle $ABC$. Let $F$ be the midpoint of the arc $BC$ of the circumcircle of triangle $BHC$, containing $H$.

Let $X$ be a point on the arc $AB$ of $\Gamma$ not containing $C$, such that $\angle AXH = \angle AFH$. Let $K$ be the circumcenter of triangle $XIA$. Prove that the lines $AO$ and $KI$ meet on $\Gamma$.

Proposed by Anant Mudgal
28 replies
Kayak
Jul 17, 2019
Ilikeminecraft
3 hours ago
Mock 22nd Thailand TMO P4
korncrazy   2
N 3 hours ago by EeEeRUT
Source: own
Let $n$ be a positive integer. In an $n\times n$ table, an upright path is a sequence of adjacent cells starting from the southwest corner to the northeast corner such that the next cell is either on the top or on the right of the previous cell. Find the smallest number of grids one needs to color in an $n\times n$ table such that there exists only one possible upright path not containing any colored cells.
2 replies
korncrazy
Yesterday at 6:53 PM
EeEeRUT
3 hours ago
NEPAL TST 2025 DAY 2
Tony_stark0094   6
N 4 hours ago by GeoKing
Consider an acute triangle $\Delta ABC$. Let $D$ and $E$ be the feet of the altitudes from $A$ to $BC$ and from $B$ to $AC$ respectively.

Define $D_1$ and $D_2$ as the reflections of $D$ across lines $AB$ and $AC$, respectively. Let $\Gamma$ be the circumcircle of $\Delta AD_1D_2$. Denote by $P$ the second intersection of line $D_1B$ with $\Gamma$, and by $Q$ the intersection of ray $EB$ with $\Gamma$.

If $O$ is the circumcenter of $\Delta ABC$, prove that $O$, $D$, and $Q$ are collinear if and only if quadrilateral $BCQP$ can be inscribed within a circle.

$\textbf{Proposed by Kritesh Dhakal, Nepal.}$
6 replies
Tony_stark0094
Saturday at 8:40 AM
GeoKing
4 hours ago
Combinatorics or algebra?
persamaankuadrat   1
N 4 hours ago by removablesingularity
Source: OSNK 2024
How many sequences of $\left(a_{1},a_{2},a_{3},a_{4},a_{5},a_{6} \right)$ where the $a_{i}$ are positive integers such that each $1 \le a_{i} \le 4$ and none of two consecutive terms when summed are equal to $4$.
1 reply
persamaankuadrat
Yesterday at 6:34 AM
removablesingularity
4 hours ago
cursed tangent is xiooix
TestX01   2
N Apr 2, 2025 by TestX01
Source: xiooix and i
Let $ABC$ be a triangle. Let $E$ and $F$ be the intersections of the $B$ and $C$ angle bisectors with the opposite sides. Let $S = (AEF) \cap (ABC)$. Let $W = SL \cap (AEF)$ where $L$ is the major $BC$ arc midpiont.
i)Show that points $S , B , C , W , E$ and $F$ are coconic on a conic $\mathcal{C}$
ii) If $\mathcal{C}$ intersects $(ABC)$ again at $T$, not equal to $B,C$ or $S$, then prove $AL$ and $ST$ concur on $(AEF)$

I will post solution in ~1 week if noone solves.
2 replies
TestX01
Feb 25, 2025
TestX01
Apr 2, 2025
cursed tangent is xiooix
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Source: xiooix and i
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TestX01
336 posts
#1
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Let $ABC$ be a triangle. Let $E$ and $F$ be the intersections of the $B$ and $C$ angle bisectors with the opposite sides. Let $S = (AEF) \cap (ABC)$. Let $W = SL \cap (AEF)$ where $L$ is the major $BC$ arc midpiont.
i)Show that points $S , B , C , W , E$ and $F$ are coconic on a conic $\mathcal{C}$
ii) If $\mathcal{C}$ intersects $(ABC)$ again at $T$, not equal to $B,C$ or $S$, then prove $AL$ and $ST$ concur on $(AEF)$

I will post solution in ~1 week if noone solves.
This post has been edited 1 time. Last edited by TestX01, Feb 25, 2025, 11:35 PM
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Mr.Sharkman
496 posts
#2
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TestX01 wrote:
I will post solution in ~1 week if noone solves.
Its been 1 1/2 months
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TestX01
336 posts
#3
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SORRY I PROMISE I WILL POST SOON :((((((
I FORGOT OK I AM XANNED
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