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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
May 1, 2025
May is an exciting month! National MATHCOUNTS is the second week of May in Washington D.C. and our Founder, Richard Rusczyk will be presenting a seminar, Preparing Strong Math Students for College and Careers, on May 11th.

Are you interested in working towards MATHCOUNTS and don’t know where to start? We have you covered! If you have taken Prealgebra, then you are ready for MATHCOUNTS/AMC 8 Basics. Already aiming for State or National MATHCOUNTS and harder AMC 8 problems? Then our MATHCOUNTS/AMC 8 Advanced course is for you.

Summer camps are starting next month at the Virtual Campus in math and language arts that are 2 - to 4 - weeks in duration. Spaces are still available - don’t miss your chance to have an enriching summer experience. There are middle and high school competition math camps as well as Math Beasts camps that review key topics coupled with fun explorations covering areas such as graph theory (Math Beasts Camp 6), cryptography (Math Beasts Camp 7-8), and topology (Math Beasts Camp 8-9)!

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[*]May 21st, 4:00pm PT/7:00pm ET, Mathcamp 2025 Qualifying Quiz Part 2 Math Jam, Problems 5 and 6, Canada/USA Mathcamp staff will discuss solutions to Problems 5 and 6 of the 2025 Mathcamp Qualifying Quiz![/list]
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0 replies
jlacosta
May 1, 2025
0 replies
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k i Adding contests to the Contest Collections
dcouchman   1
N Apr 5, 2023 by v_Enhance
Want to help AoPS remain a valuable Olympiad resource? Help us add contests to AoPS's Contest Collections.

Find instructions and a list of contests to add here: https://artofproblemsolving.com/community/c40244h1064480_contests_to_add
1 reply
dcouchman
Sep 9, 2019
v_Enhance
Apr 5, 2023
J
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k i Zero tolerance
ZetaX   49
N May 4, 2019 by NoDealsHere
Source: Use your common sense! (enough is enough)
Some users don't want to learn, some other simply ignore advises.
But please follow the following guideline:


To make it short: ALWAYS USE YOUR COMMON SENSE IF POSTING!
If you don't have common sense, don't post.


More specifically:

For new threads:


a) Good, meaningful title:
The title has to say what the problem is about in best way possible.
If that title occured already, it's definitely bad. And contest names aren't good either.
That's in fact a requirement for being able to search old problems.

Examples:
Bad titles:
- "Hard"/"Medium"/"Easy" (if you find it so cool how hard/easy it is, tell it in the post and use a title that tells us the problem)
- "Number Theory" (hey guy, guess why this forum's named that way¿ and is it the only such problem on earth¿)
- "Fibonacci" (there are millions of Fibonacci problems out there, all posted and named the same...)
- "Chinese TST 2003" (does this say anything about the problem¿)
Good titles:
- "On divisors of a³+2b³+4c³-6abc"
- "Number of solutions to x²+y²=6z²"
- "Fibonacci numbers are never squares"


b) Use search function:
Before posting a "new" problem spend at least two, better five, minutes to look if this problem was posted before. If it was, don't repost it. If you have anything important to say on topic, post it in one of the older threads.
If the thread is locked cause of this, use search function.

Update (by Amir Hossein). The best way to search for two keywords in AoPS is to input
[code]+"first keyword" +"second keyword"[/code]
so that any post containing both strings "first word" and "second form".


c) Good problem statement:
Some recent really bad post was:
[quote]$lim_{n\to 1}^{+\infty}\frac{1}{n}-lnn$[/quote]
It contains no question and no answer.
If you do this, too, you are on the best way to get your thread deleted. Write everything clearly, define where your variables come from (and define the "natural" numbers if used). Additionally read your post at least twice before submitting. After you sent it, read it again and use the Edit-Button if necessary to correct errors.


For answers to already existing threads:


d) Of any interest and with content:
Don't post things that are more trivial than completely obvious. For example, if the question is to solve $x^{3}+y^{3}=z^{3}$, do not answer with "$x=y=z=0$ is a solution" only. Either you post any kind of proof or at least something unexpected (like "$x=1337, y=481, z=42$ is the smallest solution). Someone that does not see that $x=y=z=0$ is a solution of the above without your post is completely wrong here, this is an IMO-level forum.
Similar, posting "I have solved this problem" but not posting anything else is not welcome; it even looks that you just want to show off what a genius you are.

e) Well written and checked answers:
Like c) for new threads, check your solutions at least twice for mistakes. And after sending, read it again and use the Edit-Button if necessary to correct errors.



To repeat it: ALWAYS USE YOUR COMMON SENSE IF POSTING!


Everything definitely out of range of common sense will be locked or deleted (exept for new users having less than about 42 posts, they are newbies and need/get some time to learn).

The above rules will be applied from next monday (5. march of 2007).
Feel free to discuss on this here.
49 replies
ZetaX
Feb 27, 2007
NoDealsHere
May 4, 2019
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k-triangular sets
navi_09220114   0
5 minutes ago
Source: TASIMO 2025 Day 2 Problem 6
For an integer $k\geq 1$, we call a set $\mathcal{S}$ of $n\geq k$ points in a plane $k$-triangular if no three of them lie on the same line and whenever at most $k$ (possibly zero) points are removed from $\mathcal{S}$, the convex hull of the resulting set is a non-degenerate triangle. For given positive integer $k$, find all integers $n\geq k$ such that there exists a $k$-triangular set consisting of $n$ points.

Note. A set of points in a Euclidean plane is defined to be convex if it contains the line segments connecting each pair of its points. The convex hull of a shape is the smallest convex set that contains it.
0 replies
1 viewing
navi_09220114
5 minutes ago
0 replies
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d(2025^{a_i}-1) divides a_{n+1}
navi_09220114   0
6 minutes ago
Source: TASIMO 2025 Day 2 Problem 5
Let $a_n$ be a strictly increasing sequence of positive integers such that for all positive integers $n\ge 1$
\[d(2025^{a_n}-1)|a_{n+1}.\]Show that for any positive real number $c$ there is a positive integers $N_c$ such that $a_n>n^c$ for all $n\geq N_c$.

Note. Here $d(m)$ denotes the number of positive divisors of the positive integer $m$.
0 replies
+1 w
navi_09220114
6 minutes ago
0 replies
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Polynomial with roots a_i^3 differ by 3X
navi_09220114   0
8 minutes ago
Source: TASIMO 2025 Day 2 Problem 4
Show that there are no monic polynomials $P(X)$ with real coefficients of degree $n\geq 4$ such that the following two conditions hold:

i)They have only real roots denoted by $a_1,\cdots, a_n$ (they are not necessarily distinct);

ii) The roots of the polynomial $P(X)-3X$ are $a_1^3,\cdots, a_n^3$.

Note. A polynomial is called monic if the coefficient of its leading term, i.e., the term of the highest degree is one. For example, the polynomial $P(X)=X^{100}-10X+5$ is monic since the coefficient of $X^{100}$ is one.
0 replies
navi_09220114
8 minutes ago
0 replies
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Points on a lattice path lies on a line
navi_09220114   0
13 minutes ago
Source: TASIMO 2025 Day 1 Problem 3
Let $S$ be a nonempty subset of the points in the Cartesian plane such that for each $x\in S$ exactly one of $x+(0,1)$ or $x+(1,0)$ also belongs to $S$. Prove that for each positive integer $k$ there is a line in the plane (possibly different lines for different $k$) which contains at least $k$ points of $S$.
0 replies
navi_09220114
13 minutes ago
0 replies
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Nice concurrency
navi_09220114   0
15 minutes ago
Source: TASIMO 2025 Day 1 Problem 2
Four points $A$, $B$, $C$, $D$ lie on a semicircle $\omega$ in this order with diameter $AD$, and $AD$ is not parallel to $BC$. Points $X$ and $Y$ lie on segments $AC$ and $BD$ respectively such that $BX\parallel AD$ and $CY\perp AD$. A circle $\Gamma$ passes through $D$ and $Y$ is tangent to $AD$, and intersects $\omega$ again at $Z\neq D$. Prove that the lines $AZ$, $BC$ and $XY$ are concurrent.
0 replies
navi_09220114
15 minutes ago
0 replies
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Numbers on a circle
navi_09220114   0
21 minutes ago
Source: TASIMO 2025 Day 1 Problem 1
For a given positive integer $n$, determine the smallest integer $k$, such that it is possible to place numbers $1,2,3,\dots, 2n$ around a circle so that the sum of every $n$ consecutive numbers takes one of at most $k$ values.
0 replies
navi_09220114
21 minutes ago
0 replies
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Similar triangles and cyclic quadrilaterals
tapir1729   24
N 36 minutes ago by Rayvhs
Source: TSTST 2024, problem 8
Let $ABC$ be a scalene triangle, and let $D$ be a point on side $BC$ satisfying $\angle BAD=\angle DAC$. Suppose that $X$ and $Y$ are points inside $ABC$ such that triangles $ABX$ and $ACY$ are similar and quadrilaterals $ACDX$ and $ABDY$ are cyclic. Let lines $BX$ and $CY$ meet at $S$ and lines $BY$ and $CX$ meet at $T$. Prove that lines $DS$ and $AT$ are parallel.

Michael Ren
24 replies
tapir1729
Jun 24, 2024
Rayvhs
36 minutes ago
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Another triangle
Rushil   15
N 38 minutes ago by lakshya2009
Source: Indian RMO 1991 Problem 1
Let $P$ be an interior point of a triangle $ABC$ and $AP,BP,CP$ meet the sides $BC,CA,AB$ in $D,E,F$ respectively. Show that \[ \frac{AP}{PD} = \frac{AF}{FB} + \frac{AE}{EC}. \]
Remark
15 replies
Rushil
Oct 15, 2005
lakshya2009
38 minutes ago
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Difficult combinatorics problem
shactal   5
N an hour ago by shactal
Can someone help me with this problem? Let $n\in \mathbb N^*$. We call a distribution the act of distributing the integers from $1$
to $n^2$ represented by tokens to players $A_1$ to $A_n$ so that they all have the same number of tokens in their urns.
We say that $A_i$ beats $A_j$ when, when $A_i$ and $A_j$ each draw a token from their urn, $A_i$ has a strictly greater chance of drawing a larger number than $A_j$. We then denote $A_i>A_j$. A distribution is said to be chicken-fox-viper when $A_1>A_2>\ldots>A_n>A_1$ What is $R(n)$
, the number of chicken-fox-viper distributions?
5 replies
shactal
Yesterday at 10:40 AM
shactal
an hour ago
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My Unsolved Problem
ZeltaQN2008   1
N an hour ago by Ash_the_Bash07
Let $\triangle ABC$ satisfy $AB<AC$. The circumcircle $(O)$ and the incircle $(I)$ of $\triangle ABC$ are tangent to the sides $AC,AB$ at $E,F$, respectively. The line $BI$ meets $EF$ at $M$ and intersects $AC$ at $P$, while the line $BO$ meets $CM$ at $Q$. Construct the common external tangent $\ell$ (different from $BC$) to the incircles of the triangles $PBC$ and $QBC$. Show that $\ell$ is parallel to the line $PQ$.
1 reply
ZeltaQN2008
an hour ago
Ash_the_Bash07
an hour ago
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Point inside parallelogram
BigSams   21
N an hour ago by Want-to-study-in-NTU-MATH
Source: Canadian Mathematical Olympiad - 1997 - Problem 4.
The point $O$ is situated inside the parallelogram $ABCD$ such that $\angle AOB+\angle COD=180^{\circ}$. Prove that $\angle OBC=\angle ODC$.
21 replies
BigSams
May 7, 2011
Want-to-study-in-NTU-MATH
an hour ago
J
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Geometry
MathsII-enjoy   1
N an hour ago by MathsII-enjoy
Given triangle $ABC$ inscribed in $(O)$ with $M$ being the midpoint of $BC$. The tangents at $B, C$ of $(O)$ intersect at $D$. Let $N$ be the projection of $O$ onto $AD$. On the perpendicular bisector of $BC$, take a point $K$ that is not on $(O)$ and different from M. Circle $(KBC)$ intersects $AK$ at $F$. Lines $NF$ and $AM$ intersect at $E$. Prove that $AEF$ is an isosceles triangle.
1 reply
MathsII-enjoy
May 15, 2025
MathsII-enjoy
an hour ago
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Probably a good lemma
Zavyk09   5
N an hour ago by Orzify
Source: found when solving exercises
Let $ABC$ be a triangle with circumcircle $\omega$. Arbitrary points $E, F$ on $AC, AB$ respectively. Circumcircle $\Omega$ of triangle $AEF$ intersects $\omega$ at $P \ne A$. $BE$ intersects $CF$ at $I$. $PI$ cuts $\Omega$ and $\omega$ at $K, L$ respectively. Construct parallelogram $KFRE$. Prove that $A, R, L$ are collinear.
5 replies
1 viewing
Zavyk09
Yesterday at 12:50 PM
Orzify
an hour ago
J
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D1033 : A problem of probability for dominoes 3*1
Dattier   1
N 2 hours ago by Dattier
Source: les dattes à Dattier
Let $G$ a grid of 9*9, we choose a little square in $G$ of this grid three times, we can choose three times the same.

What the probability of cover with 3*1 dominoes this grid removed by theses little squares (one, two or three) ?
1 reply
Dattier
May 15, 2025
Dattier
2 hours ago
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a combinatoric problem
xyz123456   0
Apr 18, 2025
Source: Noga Alon
$A_n=\left[n\right]^3{,}f\left(n\right)=\min \left|S\right|{,}S\ satisfy\ S\subseteq \ A_nand\ \forall \ x\in \ A_n{,}\exists \ y{,}z\in \ S\ {,}x{,}y{,}zcollinear$ $prove\ that:\exists \ c_1{,}c_2>0{,}c_1n^{\frac{6}{5}}<f\left(n\right)<c_2n^{\frac{6}{5}}\ln n$
0 replies
xyz123456
Apr 18, 2025
0 replies
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a combinatoric problem
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xyz123456
48 posts
xyz123456
#1 h  Apr 18, 2025, 12:31 PM • 1 Y
Y by kiyoras_2001
$A_n=\left[n\right]^3{,}f\left(n\right)=\min \left|S\right|{,}S\ satisfy\ S\subseteq \ A_nand\ \forall \ x\in \ A_n{,}\exists \ y{,}z\in \ S\ {,}x{,}y{,}zcollinear$ $prove\ that:\exists \ c_1{,}c_2>0{,}c_1n^{\frac{6}{5}}<f\left(n\right)<c_2n^{\frac{6}{5}}\ln n$
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