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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 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
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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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Circumcenter lie on a line.
RootofUnityfilter   1
N a few seconds ago by lolsamo
Source: my teacher
Let $ABC$ be a triangle with circumcircle $(O)$, incentre $I$. The second intersections of $AI$, $BI$, and $CI$ with $(O)$ are $X_A$, $X_B$, and $X_C$, respectively. Lines $AI$ and $BC$ intersect at $D$ and lines $BX_C$ and $CX_B$ intersect at $X$. Suppose $(XX_BX_C)$ and $(XBC)$ intersect again at $S \ne X$. Lines $BX$ and $CX$ intersect the circumcircle of triangle $SXX_A$ again at $P \ne X$ and $Q \ne X$, respectively.
Prove that the circumcentre of triangle $SID$ lies on $PQ$.
1 reply
RootofUnityfilter
20 minutes ago
lolsamo
a few seconds ago
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Sequence and prime factors
USJL   6
N 12 minutes ago by shanelin-sigma
Source: 2025 Taiwan TST Round 2 Independent Study 1-N
Let $a_0,a_1,\ldots$ be a sequence of positive integers with $a_0=1$, $a_1=2$ and
\[a_n = a_{n-1}^{a_{n-1}a_{n-2}}-1\]for all $n\geq 2$. Show that if $p$ is a prime less than $2^k$ for some positive integer $k$, then there exists $n\leq k+1$ such that $p\mid a_n$.
6 replies
USJL
Mar 26, 2025
shanelin-sigma
12 minutes ago
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ab + c + d = 3, bc + d + a = 5, cd + a + b = 2, da + b + c = 6
parmenides51   9
N 23 minutes ago by DensSv
Source: 2021 JBMO TST Bosnia and Herzegovina P1
Determine all real numbers $a, b, c, d$ for which
$$ab + c + d = 3$$$$bc + d + a = 5$$$$cd + a + b = 2$$$$da + b + c = 6$$
9 replies
parmenides51
Oct 7, 2022
DensSv
23 minutes ago
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Prime number and composite number
mingzhehu   3
N 24 minutes ago by mingzhehu
I have one topic on how to identify Prime Number and Composite Number quickly? Maybe the number is more than 100 or 1000.......!
If there are some formula that can be used to verify the number easily, it will be highly appreciated.
Does anybody has any good idea for that?

3 replies
mingzhehu
Today at 8:06 AM
mingzhehu
24 minutes ago
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Problems for v_p(n)
xytunghoanh   1
N 40 minutes ago by whwlqkd
Hello everyone. I need some easy problems for $v_p(n)$ (use in a problem for junior) to practice. Can anyone share to me?
Thanks :>
1 reply
xytunghoanh
an hour ago
whwlqkd
40 minutes ago
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Parallel Lines and Q Point
taptya17   13
N an hour ago by everythingpi3141592
Source: India EGMO TST 2025 Day 1 P3
Let $\Delta ABC$ be an acute angled scalene triangle with circumcircle $\omega$. Let $O$ and $H$ be the circumcenter and orthocenter of $\Delta ABC,$ respectively. Let $E,F$ and $Q$ be points on segments $AB,AC$ and $\omega$, respectively, such that
$$\angle BHE=\angle CHF=\angle AQH=90^\circ.$$Prove that $OQ$ and $AH$ intersect on the circumcircle of $\Delta AEF$.

Proposed by Antareep Nath
13 replies
taptya17
Dec 13, 2024
everythingpi3141592
an hour ago
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Cool combinatorial problem (grid)
Anto0110   0
an hour ago
Suppose you have an $m \cdot n$ grid with $m$ rows and $n$ columns, and each square of the grid is filled with a non-negative integer. Let $a$ be the average of all the numbers in the grid. Prove that if $m >(10a+10)^n$ the there exist two identical rows (meaning same numbers in the same order).
0 replies
Anto0110
an hour ago
0 replies
J
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Cards and combi
AlephG_64   0
an hour ago
Source: 2025 Finals Portuguese Mathematical Olympiad P6
Maria wants to solve a challenge with a deck of cards, each with a different figure. Initially, the cards are distributed randomly into two piles, not necessarily in equal parts. Maria's goal is to get all the cards into the same pile.

On each turn, Maria takes the top card from each pile and compares them. In the rule book, there's a table that indicates, for each card match, which of the two wins. Both cards are then placed on the bottom of the winning card in the order Maria chooses. The challenge ends when all the cards are in one pile.

Show that it is always possible for Maria to solve the challenge. Regardless of the initial distribution of the cards and the table in the rule book.
0 replies
AlephG_64
an hour ago
0 replies
J
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Cool number condition
AlephG_64   0
an hour ago
Source: 2025 Finals Portuguese Mathematical Olympiad P5
An integer number $n \geq 2$ is called feirense if it is possible to write on a sheet of paper some integers such that every positive divisor of $n$ less than $n$ is the difference between two numbers on the sheet, and no other positive number is.
Find all the feirense numbers.
0 replies
AlephG_64
an hour ago
0 replies
J
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four variables inequality
JK1603JK   1
N an hour ago by arqady
Source: unknown?
Prove that $$27(a^4+b^4+c^4+d^4)+148abcd\ge (a+b+c+d)^4,\ \ \forall a,b,c,d\ge 0.$$
1 reply
JK1603JK
Yesterday at 4:26 PM
arqady
an hour ago
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Geo to make por people happy
AlephG_64   0
an hour ago
Source: 2025 Finals Portuguese Mathematical Olympiad P4
Let $ABCD$ be a square with $2cm$ side length and with center $T$. A rhombus $ARTE$ is drawn where point $E$ lies on line $DC$. What is the area of $ARTE$?
0 replies
AlephG_64
an hour ago
0 replies
J
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Cool sequence problem
AlephG_64   0
an hour ago
Source: 2025 Finals Portuguese Mathematical Olympiad P3
A computer science teacher has asked his students to write a program that, given a list of $n$ numbers $a_1, a_2, ..., a_n$, calculates the list $b_1, b_2, ..., b_n$ where $b_k$ is the number of times the number $a_k$ appears in the list. So, for example, for the list $1,2,3,1$, the program returns the list $2,1,1,2$.

Next, the teacher asked Alexandre to run the program for a list of $2025$ numbers. Then he asked him to apply the program to the resulting list, and so on, until a number greater than or equal to $k$ appears in the list. Find the largest value of $k$ for which, whatever the initial list of $2025$ positive integers $a_1, a_2, ..., a_{2025}$, it is possible for Alexander to do what the teacher asked him to do.
0 replies
AlephG_64
an hour ago
0 replies
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Table with cells
RagvaloD   8
N an hour ago by Radin_
Source: All Russian Olympiad 2017,Day2,grade 9,P8
Every cell of $100\times 100$ table is colored black or white. Every cell on table border is black. It is known, that in every $2\times 2$ square there are cells of two colors. Prove, that exist $2\times 2$ square that is colored in chess order.
8 replies
RagvaloD
May 3, 2017
Radin_
an hour ago
J
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P2 Geo that most of contestants died
AlephG_64   0
an hour ago
Source: 2025 Finals Portuguese Mathematical Olympiad P2
Let $ABCD$ be a quadrilateral such that $\angle A$ and $\angle D$ are acute and $\overline{AB} = \overline{BC} = \overline{CD}$. Suppose that $\angle BDA = 30^\circ$, prove that $\angle DAC= 30^\circ$.
0 replies
AlephG_64
an hour ago
0 replies
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J
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solve system of equation
ngocthi0101   3
N Aug 31, 2012 by vanstraelen
solve system of equation:
$\begin{cases}\sqrt{3-2x^2y-x^4y^2}+x^2(1-2x^2)=y^2\\ 1 +\sqrt{1+(x-y)^2}=x^3(x^3-x-2y^2) \end{cases}.$
3 replies
ngocthi0101
Aug 6, 2012
vanstraelen
Aug 31, 2012
J
solve system of equation
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Reveal topic tags
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ngocthi0101
88 posts
ngocthi0101
#1 h  Aug 6, 2012, 10:27 AM • 2 Y
Y by Adventure10, Mango247
solve system of equation:
$\begin{cases}\sqrt{3-2x^2y-x^4y^2}+x^2(1-2x^2)=y^2\\ 1 +\sqrt{1+(x-y)^2}=x^3(x^3-x-2y^2) \end{cases}.$
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Dr Sonnhard Graubner
16100 posts
Dr Sonnhard Graubner
#2 h  Aug 6, 2012, 1:04 PM • 2 Y
Y by Adventure10, Mango247
hello, i have found $x=-1,y=-1$.
Sonnhard.
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ngocthi0101
88 posts
ngocthi0101
#3 h  Aug 6, 2012, 3:18 PM • 2 Y
Y by Adventure10, Mango247
why ? explain, please, thank u
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vanstraelen
8949 posts
vanstraelen
#5 h  Aug 31, 2012, 9:03 PM • 2 Y
Y by Adventure10, Mango247
A second solution: $x \approx -0.89725$ and $y \approx -1.2283$, numerically found.
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