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k a April Highlights and 2025 AoPS Online Class Information
jlacosta   0
Apr 2, 2025
Spring is in full swing and summer is right around the corner, what are your plans? At AoPS Online our schedule has new classes starting now through July, so be sure to keep your skills sharp and be prepared for the Fall school year! Check out the schedule of upcoming classes below.

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April 9th (Webinar), 4:00pm PT/7:00pm ET, Learn about Video-based Summer Camps at the Virtual Campus
[*]April 10th (Math Jam), 4:30pm PT/7:30pm ET, 2025 MathILy and MathILy-Er Math Jam: Multibackwards Numbers
[*]April 22nd (Webinar), 4:00pm PT/7:00pm ET, Competitive Programming at AoPS (USACO).[/list]
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0 replies
jlacosta
Apr 2, 2025
0 replies
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
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
Cardinality of sets containing both multiples and non-multiples of 3
tom-nowy   1
N 5 minutes ago by Tkn
Source: Own
Let $n$ be a positive integer, and let $N$ be the set $\{ 1,2, \ldots, n\}$.
Let the sets $X_n$ and $Y_n$ be difined as:
\begin{align*}
X_n = &\left\{ (x_1,x_2,x_3) \in N^3  \mid  x_1+x_2+x_3 \text{ is not divisible by } 3. \right\}, \\
Y_n = &\left\{ (y_1,y_2,y_3) \in  N^3  \mid  \text{Among }  y_1-y_2,\, y_2-y_3,\, y_3-y_1, 
\text{ at least one is} \right. \\ 
& \left. \text{\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:divisible by }  3  \text{ and at least one is not divisible by } 3. \right\}. 
\end{align*}Which is larger, $\left| X_n \right|$ or $\left| Y_n \right|$ ?
1 reply
tom-nowy
3 hours ago
Tkn
5 minutes ago
Constant sum
M4RI0   3
N 7 minutes ago by Rohit-2006
Source: Cono Sur Olympiad, Uruguay 1989, Problem #5
Let $ABCD$ be a square with diagonals $AC$ and $BD$, and $P$ a point in one of the sides of the square. Show that the sum of the distances from P to the diagonals is constant.
3 replies
M4RI0
May 14, 2006
Rohit-2006
7 minutes ago
Four circles
April   56
N 14 minutes ago by ihategeo_1969
Source: Canada Mathematical Olympiad 2007
Let the incircle of triangle $ ABC$ touch sides $ BC,\, CA$ and $ AB$ at $ D,\, E$ and $ F,$ respectively. Let $ \omega,\,\omega_{1},\,\omega_{2}$ and $ \omega_{3}$ denote the circumcircles of triangle $ ABC,\, AEF,\, BDF$ and $ CDE$ respectively.

Let $ \omega$ and $ \omega_{1}$ intersect at $ A$ and $ P,\,\omega$ and $ \omega_{2}$ intersect at $ B$ and $ Q,\,\omega$ and $ \omega_{3}$ intersect at $ C$ and $ R.$

$ a.$ Prove that $ \omega_{1},\,\omega_{2}$ and $ \omega_{3}$ intersect in a common point.

$ b.$ Show that $ PD,\, QE$ and $ RF$ are concurrent.
56 replies
1 viewing
April
Jul 26, 2007
ihategeo_1969
14 minutes ago
Function equation
LeDuonggg   0
18 minutes ago
Find all functions $f: \mathbb{R^+} \rightarrow \mathbb{R^+}$ , such that for all $x,y>0$:
\[ f(x+f(y))=\dfrac{f(x)}{1+f(xy)}\]
0 replies
LeDuonggg
18 minutes ago
0 replies
If ab+1 is divisible by A then so is a+b
ravengsd   1
N 27 minutes ago by NO_SQUARES
Source: Romania EGMO TST 2025 Day 2, Problem 4
Find the greatest positive integer $A$ such that, for all positive integers $a$ and $b$, if $A$ divides $ab+1$, then $A$ divides $a+b$.
1 reply
ravengsd
an hour ago
NO_SQUARES
27 minutes ago
Coincide
giangtruong13   3
N 30 minutes ago by giangtruong13
Source: Hanoi Specialized School's Math Test (Round 2 - Phase 1)
Let $ABCD$ be a trapezoid inscribed in circle $(O)$, $AD||BC, AD < BC$. Let $P$ is the symmetric point of $A$ across $BC$, $AP$ intersects $BC$ at $K$. Let $M$ is midpoint of $BC$ and $H$ is orthocenter of triangle $ABC$. On $BD$ take a point $F$ so that $AF||HM$. Prove that: $ FK,AC,PD$ coincide
3 replies
giangtruong13
Apr 27, 2025
giangtruong13
30 minutes ago
Geometry..Pls
Jackson0423   0
34 minutes ago
In equilateral triangle \( ABC \), let \( AB = 10 \). Point \( D \) lies on segment \( BC \) such that \( BC = 4 \cdot DC \). Let \( O \) and \( I \) be the circumcenter and incenter of triangle \( ABD \), respectively. Let \( O' \) and \( I' \) be the circumcenter and incenter of triangle \( ACD \), respectively. Suppose that lines \( OI \) and \( O'I' \) intersect at point \( X \). Find the length of \( XD \).
0 replies
Jackson0423
34 minutes ago
0 replies
The number of integers
Fang-jh   17
N 35 minutes ago by MathLuis
Source: ChInese TST 2009 P3
Prove that for any odd prime number $ p,$ the number of positive integer $ n$ satisfying $ p|n! + 1$ is less than or equal to $ cp^\frac{2}{3}.$ where $ c$ is a constant independent of $ p.$
17 replies
1 viewing
Fang-jh
Apr 4, 2009
MathLuis
35 minutes ago
Geometry Proof
Jackson0423   3
N 39 minutes ago by Jackson0423
In triangle \( \triangle ABC \), point \( D \) on \( AB \) satisfies \( DB = BC \) and \( \angle DCA = 30^\circ \).
Let \( X \) be the point where the perpendicular from \( B \) to line \( DC \) meets the angle bisector of \( \angle BCA \).
Then, the relation \( AD \cdot DC = BD \cdot AX \) holds.

Prove that \( \triangle ABC \) is an isosceles triangle.
3 replies
Jackson0423
Yesterday at 4:17 PM
Jackson0423
39 minutes ago
4-var inequality
RainbowNeos   2
N 41 minutes ago by nexu
Given $a,b,c,d>0$, show that
\[\frac{a}{b}+\frac{b}{c}+\frac{c}{d}+\frac{d}{a}\geq 4+\frac{8(a-c)^2}{(a+b+c+d)^2}.\]
2 replies
RainbowNeos
6 hours ago
nexu
41 minutes ago
An inequality with n of Maximum with Minimum
Qing-Cloud   1
N 44 minutes ago by Qing-Cloud
Let $ n $ be a positive integer divisible by 3, and let $ x_1, x_2, \dots, x_n $ be non-negative real numbers satisfying $ \sum_{i=1}^n x_i = 1 $. Define
\[
M_n = \min_{1 \le i \le n} \{x_i x_{3i}\},  
\]where the indices are taken modulo $ n $. Determine the maximum possible value of $ M_n $.
1 reply
Qing-Cloud
an hour ago
Qing-Cloud
44 minutes ago
NT Disguised as a Poly (GAMO P2)
Aritra12   14
N an hour ago by ihategeo_1969
Source: GAMO DAY 1 P2 (USAMO)
Find all polynomials $P(x)$ with integer coefficients which satisfy the following conditions:
[list]
[*] $P(n)$ is a positive integer for any positive integer $n$.
[*] $P(n)!|\prod_{k=1}^n\left(2^{P(k)+k-1}-2^{k-1}\right)$ for all positive integers $n$.
[/list]

$\textit{Proposed by Aritra12, TLP.39 and Orestis Lignos}$
14 replies
Aritra12
Apr 12, 2021
ihategeo_1969
an hour ago
Do not try to bash on beautiful geometry
ItzsleepyXD   4
N an hour ago by Tkn
Source: Own , Mock Thailand Mathematic Olympiad P9
Let $ABC$be triangle with point $D,E$ and $F$ on $BC,AB,CA$
such that $BE=CF$ and $E,F$ are on the same side of $BC$
Let $M$ be midpoint of segment $BC$ and $N$ be midpoint of segment $EF$
Let $G$ be intersection of $BF$ with $CE$ and $\dfrac{BD}{DC}=\dfrac{AC}{AB}$
Prove that $MN\parallel DG$
4 replies
ItzsleepyXD
Yesterday at 9:30 AM
Tkn
an hour ago
Surjective number theoretic functional equation
snap7822   1
N 2 hours ago by internationalnick123456
Source: 2025 Taiwan TST Round 3 Independent Study 2-N
Let $f:\mathbb{N} \rightarrow \mathbb{N}$ be a function satisfying the following conditions:
[list=i]
[*] For all $m, n \in \mathbb{N}$, if $m > n$ and $f(m) > f(n)$, then $f(m-n) = f(n)$;
[*] $f$ is surjective.
[/list]
Find the maximum possible value of $f(2025)$.

Proposed by snap7822
1 reply
snap7822
3 hours ago
internationalnick123456
2 hours ago
Symmetric Function Theorem
Ji Chen   2
N May 28, 2009 by lovegyz1
Source: elementary symmetric function and power sum function
Let $ x_{1},x_{2},\cdots,x_{n}$ and $ y_{1},y_{2},\cdots,y_{n}$ are two sets of nonnegative quantities. Now, if we have all the following conditions fulfilled:

$ x_{1} + x_{2} + \cdots + x_{n}\leq y_{1} + y_{2} + \cdots + y_{n},$

$ \sum_{i < j}x_{i}x_{j}\leq\sum_{i < j}y_{i}y_{j},$

$ \vdots$

$ x_{1}x_{2}\cdots x_{n}\leq y_{1}y_{2}\cdots y_{n},$

(generally the $ r$-th elementary symmetric functions $ E_{r}(x)\leq E_{r}(y)$ for any natural number $ r$ with $ 1 \leq r \leq n$)

then $ \sum_{i = 1}^{n} x_{i}^p\leq \sum_{i = 1}^{n} y_{i}^p$ for $ 0 < p < 1$.

Proof.
2 replies
Ji Chen
Mar 15, 2008
lovegyz1
May 28, 2009
Symmetric Function Theorem
G H J
G H BBookmark kLocked kLocked NReply
Source: elementary symmetric function and power sum function
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Ji Chen
827 posts
#1 • 11 Y
Y by Crazy_LittleBoy, Grotex, Havister, Adventure10, Modern_Hunter, IraeVid13, Mango247, SunnyEvan, MS_asdfgzxcvb, and 2 other users
Let $ x_{1},x_{2},\cdots,x_{n}$ and $ y_{1},y_{2},\cdots,y_{n}$ are two sets of nonnegative quantities. Now, if we have all the following conditions fulfilled:

$ x_{1} + x_{2} + \cdots + x_{n}\leq y_{1} + y_{2} + \cdots + y_{n},$

$ \sum_{i < j}x_{i}x_{j}\leq\sum_{i < j}y_{i}y_{j},$

$ \vdots$

$ x_{1}x_{2}\cdots x_{n}\leq y_{1}y_{2}\cdots y_{n},$

(generally the $ r$-th elementary symmetric functions $ E_{r}(x)\leq E_{r}(y)$ for any natural number $ r$ with $ 1 \leq r \leq n$)

then $ \sum_{i = 1}^{n} x_{i}^p\leq \sum_{i = 1}^{n} y_{i}^p$ for $ 0 < p < 1$.

Proof.
Z K Y
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lovegyz1
5 posts
#2 • 3 Y
Y by Adventure10, Mango247, and 1 other user
Ji Chen wrote:
where $ E_{r} = \sum_{1\leq i_{1} < i_{2} < \cdots < i_{r}\leq n}{a_{i_{1}}a_{i_{2}}\cdots a_{i_{r}}},(1\leq r\leq n)$.

Then $ \frac {\partial a_{i}}{\partial E_{r}} = \frac {( - 1)^{r - 1}a_{i}^{n - r}}{\prod_{j\neq i}(a_{i} - a_{j})},(1\leq i\leq n)$,
I am the beginer of the inequality. Can you explain it?
Z K Y
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lovegyz1
5 posts
#3 • 3 Y
Y by Adventure10, Mango247, and 1 other user
I am the beginer of the inequality, who will help me? :maybe:
Z K Y
N Quick Reply
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