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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
Yesterday at 11:16 PM
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)!

Be sure to mark your calendars for the following upcoming events:
[list][*]May 9th, 4:30pm PT/7:30pm ET, Casework 2: Overwhelming Evidence — A Text Adventure, a game where participants will work together to navigate the map, solve puzzles, and win! All are welcome.
[*]May 19th, 4:30pm PT/7:30pm ET, What's Next After Beast Academy?, designed for students finishing Beast Academy and ready for Prealgebra 1.
[*]May 20th, 4:00pm PT/7:00pm ET, Mathcamp 2025 Qualifying Quiz Part 1 Math Jam, Problems 1 to 4, join the Canada/USA Mathcamp staff for this exciting Math Jam, where they discuss solutions to Problems 1 to 4 of the 2025 Mathcamp Qualifying Quiz!
[*]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
1 viewing
jlacosta
Yesterday at 11:16 PM
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
Fermat points of Pentagon
Jackson0423   0
9 minutes ago
It is known that, in general, a pentagon has three Fermat points. But I'm curious—if there are exactly two Fermat points inside the pentagon, under what conditions does the distance sum reach a minimum? Can you help me?
0 replies
Jackson0423
9 minutes ago
0 replies
Two equal angles
jayme   1
N 11 minutes ago by jayme
Dear Mathlinkers,

1. ABCD a square
2. I the midpoint of AB
3. 1 the circle center at A passing through B
4. Q the point of intersection of 1 with the segment IC
5. X the foot of the perpendicular to BC from Q
6. Y the point of intersection of 1 with the segment AX
7. M the point of intersection of CY and AB.

Prove : <ACI = <IYM.

Sincerely
Jean-Louis
1 reply
jayme
Today at 6:52 AM
jayme
11 minutes ago
Symetric inequality
Nguyenhuyen_AG   0
18 minutes ago
Let $a, \, b, \, c$ are non-negative real numbers and $k \geqslant 0.$
(i) Prove that
\[\frac{a(a^2-bc)}{\sqrt{ka+b+c}} + \frac{b(b^2-ca)}{\sqrt{kb+c+a}} + \frac{c(c^2-ab)}{\sqrt{kc+a+b}} \geqslant 0.\](ii) Prove that
\[(a^2-bc)\sqrt{ka+b+c}+(b^2-ca)\sqrt{kb+c+a}+(c^2-ab)\sqrt{kc+a+b} \geqslant 0.\]
0 replies
Nguyenhuyen_AG
18 minutes ago
0 replies
Lord Evan the Reflector
whatshisbucket   23
N 19 minutes ago by bjump
Source: ELMO 2018 #3, 2018 ELMO SL G3
Let $A$ be a point in the plane, and $\ell$ a line not passing through $A$. Evan does not have a straightedge, but instead has a special compass which has the ability to draw a circle through three distinct noncollinear points. (The center of the circle is not marked in this process.) Additionally, Evan can mark the intersections between two objects drawn, and can mark an arbitrary point on a given object or on the plane.

(i) Can Evan construct* the reflection of $A$ over $\ell$?

(ii) Can Evan construct the foot of the altitude from $A$ to $\ell$?

*To construct a point, Evan must have an algorithm which marks the point in finitely many steps.

Proposed by Zack Chroman
23 replies
whatshisbucket
Jun 28, 2018
bjump
19 minutes ago
4 variables with quadrilateral sides 2
mihaig   6
N 26 minutes ago by mihaig
Source: Own
Let $a,b,c,d\geq0$ satisfying
$$\frac1{a+1}+\frac1{b+1}+\frac1{c+1}+\frac1{d+1}=2.$$Prove
$$\left(a+b+c+d-2\right)^2+8\geq3\left(abc+abd+acd+bcd\right).$$
6 replies
mihaig
Apr 29, 2025
mihaig
26 minutes ago
Hard inequality
ys33   6
N 26 minutes ago by mihaig
Let $a, b, c, d>0$. Prove that
$\sqrt[3]{ab}+ \sqrt[3]{cd} < \sqrt[3]{(a+b+c)(b+c+d)}$.
6 replies
ys33
Today at 9:36 AM
mihaig
26 minutes ago
Nice one
imnotgoodatmathsorry   2
N 26 minutes ago by imnotgoodatmathsorry
Source: Own
With $x,y,z >0$.Prove that: $\frac{xy}{4y+4z+x} + \frac{yz}{4z+4x+y} +\frac{zx}{4x+4y+z} \le \frac{x+y+z}{9}$
2 replies
1 viewing
imnotgoodatmathsorry
2 hours ago
imnotgoodatmathsorry
26 minutes ago
analysis
Hello_Kitty   3
N 30 minutes ago by paxtonw
Prove or disprove:
1) If $u_n\geq 0$ such $\sum u_n $ converges then there is some $w_n\geq 0$ such that
$w_n\longrightarrow \infty $ and $\sum u_nw_n$ converges.
2) If $u_n\geq 0$ such $\sum u_n $ converges then there is some $w_n\geq 0$ such that
$\forall a\in\mathbb{R}, \; n^aw_n\longrightarrow \infty $ and $\sum u_nw_n$ converges .
3 replies
Hello_Kitty
2 hours ago
paxtonw
30 minutes ago
Putnam 1958 November B6
sqrtX   3
N 32 minutes ago by centslordm
Source: Putnam 1958 November
Let a complete oriented graph on $n$ points be given. Show that the vertices can be enumerated as $v_1 , v_2 ,\ldots, v_n$ such that $v_1 \rightarrow v_2 \rightarrow \cdots \rightarrow v_n.$
3 replies
sqrtX
Jul 19, 2022
centslordm
32 minutes ago
Permutation with Matrices
SomeonecoolLovesMaths   0
33 minutes ago
Consider all $n \times n$ matrix such that $\forall$ $k \leq n$, $( a_{1k}, a_{2k}, \cdots, a_{nk} )$ is a permutation of $(1,2, \cdots, n)$, call such matrices $\textit{rowgood}$. Consider all $n \times n$ matrix such that $\forall$ $k \leq n$, $( a_{k1}, a_{k2}, \cdots, a_{kn} )$ is a permutation of $(1,2, \cdots, n)$, call such matrices $\textit{columngood}$. How many $n \times n$ matrices exist that are both $\textit{rowgood}$ and $\textit{columngood}$?
0 replies
SomeonecoolLovesMaths
33 minutes ago
0 replies
Points U,V,F,E are concyclic (GAMO P5)
Aritra12   4
N 34 minutes ago by ihategeo_1969
Source: GAMO day 2 P5
Let $ABC$ be an acute, non-isosceles triangle, $AD,BE,CF$ be its heights and $(c)$ its circumcircle. $FE$ cuts the circumcircle at points $S,T$, with point $F$ being between points $S,E$. In addition, let $P,Q$ be the midpoints of the major and the minor arc $BC$, respectively. Line $DQ$ cuts $(c)$ at $R$. The circumcircles of triangles $RSF,TER,SFP$ and $TEP$ cut again $PR$ at points $X,Y,Z$ and $W$, respectively. Suppose $(\ell)$ is the line passing through the circumcenters of triangles $AXW,AYZ$ and $(\ell_B ),(\ell_C)$ the parallel lines through $B,C$ to $(\ell)$. If $(\ell_B)$ meets $CF$ at $U$ and $(\ell_C )$ meets $BE$ at $V$, then prove that points $U,V,F,E$ are concyclic.

$\textit{Proposed by Orestis Lignos}$
4 replies
Aritra12
Apr 12, 2021
ihategeo_1969
34 minutes ago
Min and Max
giangtruong13   1
N 37 minutes ago by imnotgoodatmathsorry
Source: PTNK-HCM Specialized School's Practical Math Test (Round 1)
Let $a,b \geq 0$ such that: $a^3+b^3=2$.Prove that: $$  \sqrt[3]{4} \geq a^2-ab+b^2 \geq 1$$
1 reply
giangtruong13
an hour ago
imnotgoodatmathsorry
37 minutes ago
D1023 : MVT 2.0
Dattier   1
N an hour ago by Dattier
Source: les dattes à Dattier
Let $f \in C(\mathbb R)$ derivable on $\mathbb R$ with $$\forall x \in \mathbb R,\forall h \geq 0, f(x)-3f(x+h)+3f(x+2h)-f(x+3h) \geq 0$$
Is it true that $$\forall (a,b) \in\mathbb R^2, |f(a)-f(b)|\leq \max\left(\left|f'\left(\dfrac{a+b} 2\right)\right|,\dfrac {|f'(a)+f'(b)|}{2}\right)\times |a-b|$$
1 reply
Dattier
Apr 29, 2025
Dattier
an hour ago
System of two matrices of the same rank
Assassino9931   2
N an hour ago by GreenKeeper
Source: Vojtech Jarnik IMC 2025, Category II, P2
Let $A,B$ be two $n\times n$ complex matrices of the same rank, and let $k$ be a positive integer. Prove that $A^{k+1}B^k = A$ if and only if $B^{k+1}A^k = B$.
2 replies
Assassino9931
Today at 1:02 AM
GreenKeeper
an hour ago
Soviet Union University Mathematical Contest
geekmath-31   1
N Apr 19, 2025 by Filipjack
Given a n*n matrix A, prove that there exists a matrix B such that ABA = A

Solution: I have submitted the attachment

The answer is too symbol dense for me to understand the answer.
What I have undertood:

There is use of direct product in the orthogonal decomposition. The decomposition is made with kernel and some T (which the author didn't mention) but as per orthogonal decomposition it must be its orthogonal complement.

Can anyone explain the answer in much much more detail with less use of symbols ( you can also use symbols but clearly define it).

Also what is phi | T ?
1 reply
geekmath-31
Apr 19, 2025
Filipjack
Apr 19, 2025
Soviet Union University Mathematical Contest
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geekmath-31
29 posts
#1
Y by
Given a n*n matrix A, prove that there exists a matrix B such that ABA = A

Solution: I have submitted the attachment

The answer is too symbol dense for me to understand the answer.
What I have undertood:

There is use of direct product in the orthogonal decomposition. The decomposition is made with kernel and some T (which the author didn't mention) but as per orthogonal decomposition it must be its orthogonal complement.

Can anyone explain the answer in much much more detail with less use of symbols ( you can also use symbols but clearly define it).

Also what is phi | T ?
Attachments:
PRoblem.docx (53kb)
Z K Y
The post below has been deleted. Click to close.
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Filipjack
872 posts
#2
Y by
In case it helps, here is an alternative solution: https://artofproblemsolving.com/community/q1h3249198.
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