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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
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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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FE with a lot of terms
MrHeccMcHecc   1
N an hour ago by jasperE3
Determine all functions $f : \mathbb{R} \rightarrow \mathbb{R}$ such that for all $x,y \in \mathbb{R}$ $$f(x)f(y)+f(x+y)=xf(y)+yf(x)+f(xy)+x+y+1$$
1 reply
MrHeccMcHecc
3 hours ago
jasperE3
an hour ago
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APMO 2016: Great triangle
shinichiman   26
N an hour ago by ray66
Source: APMO 2016, problem 1
We say that a triangle $ABC$ is great if the following holds: for any point $D$ on the side $BC$, if $P$ and $Q$ are the feet of the perpendiculars from $D$ to the lines $AB$ and $AC$, respectively, then the reflection of $D$ in the line $PQ$ lies on the circumcircle of the triangle $ABC$. Prove that triangle $ABC$ is great if and only if $\angle A = 90^{\circ}$ and $AB = AC$.

Senior Problems Committee of the Australian Mathematical Olympiad Committee
26 replies
shinichiman
May 16, 2016
ray66
an hour ago
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Intermediate Counting
RenheMiResembleRice   3
N an hour ago by Nab-Mathgic
A coin is flipped, a 6-sided die numbered 1 through 6 is rolled, and a 10-sided die numbered 0
through 9 is rolled. What is the probability that the coin comes up heads and the sum of the
numbers that show on the dice is 8?
3 replies
RenheMiResembleRice
Today at 7:46 AM
Nab-Mathgic
an hour ago
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Chessboard pattern
BR1F1SZ   2
N an hour ago by Ubfo
Source: 2018 Argentina TST P3
In a $100 \times 100$ board, each square is colored either white or black, with all the squares on the border of the board being black. Additionally, no $2 \times 2$ square within the board has all four squares of the same color. Prove that the board contains a $2 \times 2$ square colored like a chessboard.
2 replies
BR1F1SZ
Dec 27, 2024
Ubfo
an hour ago
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IMO ShortList 2001, geometry problem 2
orl   48
N an hour ago by legogubbe
Source: IMO ShortList 2001, geometry problem 2
Consider an acute-angled triangle $ABC$. Let $P$ be the foot of the altitude of triangle $ABC$ issuing from the vertex $A$, and let $O$ be the circumcenter of triangle $ABC$. Assume that $\angle C \geq \angle B+30^{\circ}$. Prove that $\angle A+\angle COP < 90^{\circ}$.
48 replies
orl
Sep 30, 2004
legogubbe
an hour ago
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Excalibur Identity
jjsunpu   12
N 2 hours ago by SomeonecoolLovesMaths
proof is below
12 replies
jjsunpu
Apr 3, 2025
SomeonecoolLovesMaths
2 hours ago
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Projective Electrostatistics
drago.7437   0
2 hours ago
Source: Own
Given two charges of any magnitude , a third charge collinear with them , exists such that it is in equillibirum , Prove that if a fourth charge in the same line exists such that it is in equillibrium , then the 3rd charge and the fourth charge are harmonic conjugates with respect to the two fixed charges . , For example if two +q charges are fixed then if in their midpoint placed a charge -q , it is in equillibrium , also if the same charge -q is placed at infinity the system is again in equillibrium , and the midpoint and the point at infinity are harmonic conjugates .
0 replies
drago.7437
2 hours ago
0 replies
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A very nice inequality
KhuongTrang   3
N 2 hours ago by Mathdreams
Source: own
Problem. Let $a,b,c\in \mathbb{R}:\ a+b+c=3.$ Prove that $$\color{black}{\sqrt{5a^{2}-ab+5b^{2}}+\sqrt{5b^{2}-bc+5c^{2}}+\sqrt{5c^{2}-ca+5a^{2}}\le 2(a^2+b^2+c^2)+ab+bc+ca.}$$When does equality hold?
3 replies
KhuongTrang
3 hours ago
Mathdreams
2 hours ago
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Inequalities
nhathhuyyp5c   3
N 2 hours ago by mathprodigy2011
Prove that for all positive real numbers \( a, b, c \), the following inequality holds:

\[ \sqrt{a + b} + \sqrt{b + c} + \sqrt{c + a} \geq \frac{4(ab + bc + ca)}{\sqrt{(a + b)(b + c)(c + a)}} \]
3 replies
nhathhuyyp5c
Today at 4:45 AM
mathprodigy2011
2 hours ago
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Find an angle
socrates   3
N 2 hours ago by Nari_Tom
Source: Baltic Way 2016, Problem 18
Let $ABCD$ be a parallelogram such that $\angle BAD = 60^{\circ}.$ Let $K$ and $L$ be the midpoints of $BC$ and $CD,$ respectively. Assuming that $ABKL$ is a cyclic quadrilateral, find $\angle ABD.$
3 replies
socrates
Nov 5, 2016
Nari_Tom
2 hours ago
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Inequalities
sqing   2
N 2 hours ago by DAVROS
Let $a,b$ be real numbers such that $ a^2+b^2+a^3 +b^3=4 . $ Prove that
$$a+b \leq 2$$Let $a,b$ be real numbers such that $a+b + a^2+b^2+a^3 +b^3=6 . $ Prove that
$$a+b \leq 2$$
2 replies
sqing
Yesterday at 1:10 PM
DAVROS
2 hours ago
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ISI 2019 : Problem #7
integrated_JRC   12
N 2 hours ago by Levieee
Source: I.S.I. 2019
Let $f$ be a polynomial with integer coefficients. Define $$a_1 = f(0)~,~a_2 = f(a_1) = f(f(0))~,$$and $~a_n = f(a_{n-1})$ for $n \geqslant 3$.

If there exists a natural number $k \geqslant 3$ such that $a_k = 0$, then prove that either $a_1=0$ or $a_2=0$.
12 replies
integrated_JRC
May 5, 2019
Levieee
2 hours ago
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Problem 1 of the HMO 2025
GreekIdiot   4
N 2 hours ago by eric201291
Let $P(x)=x^4+5x^3+mx^2+5nx+4$ have $2$ distinct real roots, which sum up to $-5$. If $m,n \in \mathbb {Z_+}$, find the values of $m,n$ and their corresponding roots.
4 replies
GreekIdiot
Feb 22, 2025
eric201291
2 hours ago
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Nordic 2025 P1
anirbanbz   5
N 2 hours ago by eric201291
Source: Nordic 2025
Let $n$ be a positive integer greater than $2$. Find all functions $f: \mathbb{Z} \rightarrow \mathbb{Z}$ satisfying:
$(f(x+y))^{n} = f(x^{n})+f(y^{n}),$ for all integers $x,y$
5 replies
anirbanbz
Mar 25, 2025
eric201291
2 hours ago
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perimeter of triangle with integer sides 2012 Estonia Open Junior 2.5
parmenides51   2
N Oct 8, 2020 by hardsinc
Is it possible that the perimeter of a triangle whose side lengths are integers, is divisible by the double of the longest side length?
2 replies
parmenides51
Oct 8, 2020
hardsinc
Oct 8, 2020
J
perimeter of triangle with integer sides 2012 Estonia Open Junior 2.5
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Reveal topic tags
geometry
perimeter
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parmenides51
30629 posts
parmenides51
#1 h  Oct 8, 2020, 1:06 AM
Y by
Is it possible that the perimeter of a triangle whose side lengths are integers, is divisible by the double of the longest side length?
Z K Y
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natmath
8219 posts
natmath
#2 h  Oct 8, 2020, 3:09 AM
Y by
We have the system
$a+b=(2k-1)c$ for some natural $k$
$a,b<c$
$a+b>c$

Since each of $a$ and $b$ are less than $c$, their sum must be less than $2c$, so $c<a+b<2c$
Since $a+b$ can never be a multiple of $c$, it can also never be an odd multiple of $c$
Z K Y
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hardsinc
37 posts
hardsinc
#3 h  Oct 8, 2020, 4:37 AM
Y by
Let a,b,c with the sides of the triangle
W.L.O.G assume c as the longest side
By triangle inequality a+b>c
a+b+c>2c
If 2c divides a+b+c
a+b+c=4c.. minimum
a+b=3c
but c is longest side
Hence Contradiction..thus no such traingle exist
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