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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.

WOOT early bird pricing is in effect, don’t miss out! If you took MathWOOT Level 2 last year, no worries, it is all new problems this year! Our Worldwide Online Olympiad Training program is for high school level competitors. AoPS designed these courses to help our top students get the deep focus they need to succeed in their specific competition goals. Check out the details at this link for all our WOOT programs in math, computer science, chemistry, and physics.

Looking for summer camps in math and language arts? Be sure to check out the video-based summer camps offered at the Virtual Campus that are 2- to 4-weeks in duration. 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)!

Be sure to mark your calendars for the following events:
[list][*]April 3rd (Webinar), 4pm PT/7:00pm ET, Learning with AoPS: Perspectives from a Parent, Math Camp Instructor, and University Professor
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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
Scary Binomial Coefficient Sum
EpicBird08   40
N 2 minutes ago by Irreplaceable
Source: USAMO 2025/5
Determine, with proof, all positive integers $k$ such that $$\frac{1}{n+1} \sum_{i=0}^n \binom{n}{i}^k$$is an integer for every positive integer $n.$
40 replies
EpicBird08
Mar 21, 2025
Irreplaceable
2 minutes ago
maximum profit
Ecrin_eren   0
23 minutes ago
In a meeting attended by 20 businessmen, some of them know each other and do business only with the people they know. The participants are numbered from 1 to 20 according to the order in which they arrived. Let aₖ represent the number of people that person number k knows. (For example, if person 5 knows 9 people, then a₅ = 9.)

If person k knows person n, then the profit that k earns from doing business with n is:

(1 / aₖ) + (1 / aₙ) + (1 / (aₖ × aₙ))

What is the maximum total profit that any participant in this meeting can earn?
0 replies
+1 w
Ecrin_eren
23 minutes ago
0 replies
GCD of sums of consecutive divisors
Lukaluce   2
N 24 minutes ago by Tintarn
Source: EGMO 2025 P1
For a positive integer $N$, let $c_1 < c_2 < ... < c_m$ be all the positive integers smaller than $N$ that are coprime to $N$. Find all $N \ge 3$ such that
\[gcd(N, c_i + c_{i + 1}) \neq 1\]for all $1 \le i \le m - 1$.
2 replies
+3 w
Lukaluce
an hour ago
Tintarn
24 minutes ago
AD=BE implies ABC right
v_Enhance   113
N 26 minutes ago by LeYohan
Source: European Girl's MO 2013, Problem 1
The side $BC$ of the triangle $ABC$ is extended beyond $C$ to $D$ so that $CD = BC$. The side $CA$ is extended beyond $A$ to $E$ so that $AE = 2CA$. Prove that, if $AD=BE$, then the triangle $ABC$ is right-angled.
113 replies
v_Enhance
Apr 10, 2013
LeYohan
26 minutes ago
Classic 3 variable inequality
AndreiVila   4
N 26 minutes ago by Rohit-2006
Source: Mathematical Minds 2024 P4
Let $a$, $b$, $c$ be positive real numbers such that $a+b+c=3$. Prove that $$\sqrt[3]{\frac{a^3+b^3}{2}}+\sqrt[3]{\frac{b^3+c^3}{2}}+\sqrt[3]{\frac{c^3+a^3}{2}}\leqslant a^2+b^2+c^2.$$
Proposed by Andrei Vila
4 replies
AndreiVila
Sep 29, 2024
Rohit-2006
26 minutes ago
pairwise coprime sum gcd
InterLoop   2
N 31 minutes ago by lelouchvigeo
Source: EGMO 2025/1
For a positive integer $N$, let $c_1 < c_2 < \dots < c_m$ be all the positive integers smaller than $N$ that are coprime to $N$. Find all $N \ge 3$ such that
$$\gcd(N, c_i + c_{i+1}) \neq 1$$for all $1 \le i \le m - 1$.
2 replies
+2 w
InterLoop
2 hours ago
lelouchvigeo
31 minutes ago
Inequalities
hn111009   0
an hour ago
Source: Maybe anywhere?
Let $a,b,c>0;r,s\in\mathbb{R}$ satisfied $a+b+c=1.$ Find minimum and maximum of $$P=a^rb^s+b^rc^s+c^ra^s.$$
0 replies
hn111009
an hour ago
0 replies
sequence infinitely similar to central sequence
InterLoop   1
N an hour ago by stmmniko
Source: EGMO 2025/2
An infinite increasing sequence $a_1 < a_2 < a_3 < \dots$ of positive integers is called central if for every positive integer $n$, the arithmetic mean of the first $a_n$ terms of the sequence is equal to $a_n$.

Show that there exists an infinite sequence $b_1$, $b_2$, $b_3$, $\dots$ of positive integers such that for every central sequence $a_1$, $a_2$, $a_3$, $\dots$, there are infinitely many positive integers $n$ with $a_n = b_n$.
1 reply
+2 w
InterLoop
2 hours ago
stmmniko
an hour ago
Three concyclic quadrilaterals
Lukaluce   1
N an hour ago by InterLoop
Source: EGMO 2025 P3
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 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. $\newline$
The orthocentre of a triangle is the point of intersection of its altitudes.
1 reply
Lukaluce
an hour ago
InterLoop
an hour ago
inqualities
pennypc123456789   0
an hour ago
Given positive real numbers \( x \) and \( y \). Prove that:
\[
\frac{1}{x} + \frac{1}{y} + 2 \sqrt{\frac{2}{x^2 + y^2}} + 4 \geq 4 \left( \sqrt{\frac{2}{x^2 + 1}} + \sqrt{\frac{2}{y^2 + 1}} \right).
\]
0 replies
pennypc123456789
an hour ago
0 replies
postaffteff
JetFire008   18
N 2 hours ago by Captainscrubz
Source: Internet
Let $P$ be the Fermat point of a $\triangle ABC$. Prove that the Euler line of the triangles $PAB$, $PBC$, $PCA$ are concurrent and the point of concurrence is $G$, the centroid of $\triangle ABC$.
18 replies
JetFire008
Mar 15, 2025
Captainscrubz
2 hours ago
SMT online indvidual tests
techb   4
N 2 hours ago by techb
Hi, are you all able to see both of the individual subject tests on your comp.mt for SMT? I can't see mine as I have registered for algebra and calculus, but I only see algebra. (See attachment)

Please respond fast.
4 replies
+1 w
techb
3 hours ago
techb
2 hours ago
Bashtastic
nosysnow   58
N 3 hours ago by AshAuktober
Source: 2018 AIME 1 #11
Find the least positive integer $n$ such that when $3^n$ is written in base $143$, its two right-most digits in base $143$ are $01$.
58 replies
nosysnow
Mar 7, 2018
AshAuktober
3 hours ago
PROM^2 for Girls 2025
mathisfun17   25
N Today at 7:05 AM by Yiyj1
Hi everyone!

The Princeton International School of Math and Science (PRISMS) Math Team is delighted that $PROM^2$ for Girls, PRISMS Online Math Meet for Girls, is happening this spring! https://www.prismsus.org/events/prom/home/index

We warmly invite all middle school girls to join us! This is a fantastic opportunity for young girls to connect with others interested in math as well as prepare for future math contests.

This contest will take place online from 12:00 pm to 3:00 pm EST on Saturday, April 26th, 2025.

The competition will include a team and individual round as well as activities like origami. You can see a detailed schedule here. https://prismsus.org/events/prom/experience/schedule.

Registration is FREE, there are cash prizes for participants who place in the top 10 and cool gifts for all participants.

1st place individual: $500 cash
2nd place individual: $300 cash
3rd place individual: $100 cash
4th-10th place individual: $50 cash each

Some FAQs:
Q: How difficult are the questions?
A: The problem difficulty is around AMC 8 or Mathcounts level.

Q: Are there any example problems?
A: You can find some archived here: https://www.prismsus.org/events/prom/achieve/achieve

Registration is open now. https://www.prismsus.org/events/prom/register/register. Email us at prom2@prismsus.org with any questions.

The PRISMS Peregrines Math Team welcomes you!
25 replies
mathisfun17
Feb 22, 2025
Yiyj1
Today at 7:05 AM
9c4 sperner's theorem
ccarolyn4   26
N Dec 3, 2024 by EaZ_Shadow
Source: 2024 AMC 10 P12
A group of $100$ students from different countries meet at a mathematics competition. Each student speaks the same number of languages, and, for every pair of students $A$ and $B$, student $A$ speaks some language that student $B$ does not speak, and student $B$ speaks some language that student $A$ does not speak. What is the least possible total number of languages spoken by all the students?

$
\textbf{(A) }9 \qquad
\textbf{(B) }10 \qquad
\textbf{(C) }12 \qquad
\textbf{(D) }51 \qquad
\textbf{(E) }100 \qquad
$
26 replies
ccarolyn4
Nov 13, 2024
EaZ_Shadow
Dec 3, 2024
9c4 sperner's theorem
G H J
Source: 2024 AMC 10 P12
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ccarolyn4
24 posts
#1 • 3 Y
Y by Soccerstar9, OronSH, akliu
A group of $100$ students from different countries meet at a mathematics competition. Each student speaks the same number of languages, and, for every pair of students $A$ and $B$, student $A$ speaks some language that student $B$ does not speak, and student $B$ speaks some language that student $A$ does not speak. What is the least possible total number of languages spoken by all the students?

$
\textbf{(A) }9 \qquad
\textbf{(B) }10 \qquad
\textbf{(C) }12 \qquad
\textbf{(D) }51 \qquad
\textbf{(E) }100 \qquad
$
This post has been edited 3 times. Last edited by ccarolyn4, Dec 23, 2024, 11:31 PM
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ccarolyn4
24 posts
#2 • 6 Y
Y by megarnie, solasky, Soccerstar9, lprado, Turtwig113, zhoujef000
confirmed 9 (A) from cm

my fav question from test <3
This post has been edited 2 times. Last edited by ccarolyn4, Nov 13, 2024, 5:21 PM
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gracemoon124
872 posts
#3
Y by
yeah what you do is $\tbinom nk$ where n is the number of languages in total & k is the number of languages each student speaks, we need $\tbinom nk \ge 100$, so we test out answer choices lol
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gladIasked
632 posts
#4 • 1 Y
Y by Andyluo
this is a really nice problem; unironically my favorite problem on the test
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paganiniana
210 posts
#5
Y by
9 choose 4 satisfies already, so A
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Squidget
433 posts
#6
Y by
I almost answered 100 but I came to my senses and I decided to leave unanswered. I only had 30 sec left anyway
This post has been edited 1 time. Last edited by Squidget, Nov 13, 2024, 5:25 PM
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zlrara01
335 posts
#7
Y by
this was a really cool problem, I just noticed it sounded like a mathcounts problem and did 9c4
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pingpongmerrily
3558 posts
#8
Y by
it was cool but then 9c4 seemed to easy so i quadruple checked it lol
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KevinYang2.71
413 posts
#9 • 5 Y
Y by solasky, LostDreams, OronSH, ranu540, Alex-131
this is why you read diestel
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MathRook7817
650 posts
#10
Y by
bro i skipped this it was easy
lol i sold
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aleyang
192 posts
#11
Y by
This was worded kinda weirdly but was just testing answer choices once you knew what it was saying
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Elephant200
1471 posts
#12
Y by
This is a nice problem, but of course I blanked on the test and I had no idea how to do it. Then 5 minutes after it ends I figure it out :wallbash_red:
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Nayantara5
64 posts
#13
Y by
Im really confused as to why it's not 100. Doesn't every student have to speak a language that 99 students don't speak?
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zlrara01
335 posts
#14
Y by
Nayantara5 wrote:
Im really confused as to why it's not 100. Doesn't every student have to speak a language that 99 students don't speak?

No
Essentially, if you choose any two people out of the 100, then one person has to know a language that the other person does not know
This post has been edited 1 time. Last edited by zlrara01, Nov 13, 2024, 7:05 PM
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wuwang2002
1203 posts
#15
Y by
i think the problem statement could have been closer (makes me think it asks for the number of languages every single one of them speaks)
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orangebear
663 posts
#16
Y by
I am so dumb bruh, my brain just won’t work during the test.
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lpieleanu
2895 posts
#17 • 1 Y
Y by ranu540
Solution
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akliu
1780 posts
#18 • 1 Y
Y by Turtwig113
Oh man this was my favorite question on the test despite it being a combo question. This is what MAA needs to put on their tests.

Denote the set of total languages as $S$. Any two subsets $A$ and $B$ such that $A, B \in S$ and $|A| = |B|$ are guaranteed to satisfy the problem condition as long as $A \neq B$. Then, we just need to find the smallest value of $n$ such that for some $k$, $\tbinom{n}{k} \geq 100$. Manually checking the answer choices, $9$ works.
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saturnrocket
1306 posts
#19 • 1 Y
Y by Sabburi
9 took me 20 minutes to find out what it was saying lol
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CuriousMathBoy72
786 posts
#20
Y by
I somewhat grasped the idea of the problem first time (thoguht it was $\binom{n}2$ at first) then I read the problem and it was $\binom{n}{k}$ so I just made an entire pascals triangle
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RabtejKalra
61 posts
#21
Y by
Why did I say 12?
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Tetra_scheme
90 posts
#22
Y by
Another good one. Note that if they have the same number of languages, it is sufficient for each person to have a different total configuration of languages. This means n choose k is greater than 100 so 9 is the smallest.
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Facejo
2848 posts
#23
Y by
This was a really fun problem and probably my second favorite but it would have been even better if 9 wasn't the smallest option.
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jocaleby1
195 posts
#24
Y by
I feel like this problem could have been #5 - 10 if it weren't for the wording that made it a bit confusing. This problem felt really easy, so I kept second guessing myself. Overall, I liked this problem.

edit: 100th post :)
This post has been edited 1 time. Last edited by jocaleby1, Nov 16, 2024, 9:08 PM
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EaZ_Shadow
1197 posts
#25
Y by
Elephant200 wrote:
This is a nice problem, but of course I blanked on the test and I had no idea how to do it. Then 5 minutes after it ends I figure it out :wallbash_red:

I did it with one minute remaining lol
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youngjin
198 posts
#26
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Im pretty sure the fact they all speak the same number of languages is irrelevant as $9$ is the smallest either way
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EaZ_Shadow
1197 posts
#27 • 1 Y
Y by WHO_LET_ME_COOK
ccarolyn4 wrote:
A group of $100$ students from different countries meet at a mathematics competition. Each student speaks the same number of languages, and, for every pair of students $A$ and $B$, student $A$ speaks some language that student $B$ does not speak, and student $B$ speaks some language that student $A$ does not speak. What is the least possible total number of languages spoken by all the students?

$
\textbf{(A) }9 \qquad
\textbf{(B) }10 \qquad
\textbf{(C) }12 \qquad
\textbf{(D) }51 \qquad
\textbf{(E) }100 \qquad
$

Misplace honestly shoulda been 16 or smth it tricked so many people
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