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Cyclic ine
m4thbl3nd3r   2
N 6 minutes ago by m4thbl3nd3r
Let $a,b,c>0$ such that $a+b+c=3$. Prove that $$a^3b+b^3c+c^3a+9abc\le 12$$
2 replies
m4thbl3nd3r
Yesterday at 3:17 PM
m4thbl3nd3r
6 minutes ago
Sharygin 2025 CR P14
Gengar_in_Galar   9
N 13 minutes ago by SimplisticFormulas
Source: Sharygin 2025
A point $D$ lies inside a triangle $ABC$ on the bisector of angle $B$. Let $\omega_{1}$ and $\omega_{2}$ be the circles touching $AD$ and $CD$ at $D$ and passing through $B$; $P$ and $Q$ be the common points of $\omega_{1}$ and $\omega_{2}$ with the circumcircle of $ABC$ distinct from $B$. Prove that the circumcircles of the triangles $PQD$ and $ACD$ are tangent.
Proposed by: L Shatunov
9 replies
Gengar_in_Galar
Mar 10, 2025
SimplisticFormulas
13 minutes ago
A cyclic inequality
JK1603JK   0
29 minutes ago
Source: unknown
Let a,b,c be real numbers. Prove that a^6+b^6+c^6\ge 2(a+b+c)(ab+bc+ca)(a-b)(b-c)(c-a).
0 replies
JK1603JK
29 minutes ago
0 replies
IMO 2024 Prediction
GreenTea2593   100
N 32 minutes ago by ohiorizzler1434
Source: inspired by math90
Hello Aops! Since IMO 2024 is less than a week away,
What are your predictions for the category of each problem at IMO 2024?

If you want to write your prediction, please write it in the form ABC DEF
Where A,B,C,D,E,F are problems 1,2,3,4,5,6 respectively. Each letter should be A,C,G or N.

Rules:
1. Problems 1,2,4,5 are distinct categories.
2. Each day consists of 3 distinct categories.

Edit : the answer is ANC GCA
100 replies
GreenTea2593
Jul 10, 2024
ohiorizzler1434
32 minutes ago
Mysterious 42
steven_zhang123   0
an hour ago
Source: China TST 2001 Quiz 5 P3
Consider the problem of expressing $42$ as \(42 = x^3 + y^3 + z^3 - w^2\), where \(x, y, z, w\) are integers. Determine the number of ways to represent $42$ in this form and prove your conclusion.
0 replies
steven_zhang123
an hour ago
0 replies
Maximizing
steven_zhang123   0
an hour ago
Source: China TST 2001 Quiz 5 P2
Find the largest positive real number \( c \) such that for any positive integer \( n \), satisfies \(\{ \sqrt{7n} \} \geq \frac{c}{\sqrt{7n}}\).
0 replies
1 viewing
steven_zhang123
an hour ago
0 replies
Quality FE
pablock   36
N an hour ago by ehuseyinyigit
Source: 2020 Iberoamerican #5
Determine all functions $f: \mathbb{R} \rightarrow \mathbb{R}$ such that $$f(xf(x-y))+yf(x)=x+y+f(x^2),$$for all real numbers $x$ and $y.$
36 replies
pablock
Nov 17, 2020
ehuseyinyigit
an hour ago
Problem 4 (second day)
darij grinberg   91
N 2 hours ago by Marcus_Zhang
Source: IMO 2004 Athens
Let $n \geq 3$ be an integer. Let $t_1$, $t_2$, ..., $t_n$ be positive real numbers such that \[n^2 + 1 > \left( t_1 + t_2 + \cdots + t_n \right) \left( \frac{1}{t_1} + \frac{1}{t_2} + \cdots + \frac{1}{t_n} \right).\] Show that $t_i$, $t_j$, $t_k$ are side lengths of a triangle for all $i$, $j$, $k$ with $1 \leq i < j < k \leq n$.
91 replies
darij grinberg
Jul 13, 2004
Marcus_Zhang
2 hours ago
Whiteboard magic again
navi_09220114   2
N 2 hours ago by mashumaro
Source: Malaysian IMO TST 2025 P5
Fix positive integers $n$ and $k$, and $2n$ positive (not neccesarily distinct) real numbers $a_1,\cdots, a_n$, $b_1, \cdots, b_n$. An equation is written on a whiteboard: $$t=*\times*\times\cdots\times*$$where $t$ is a fixed positive real number, with exactly $k$ asterisks.

Ebi fills each asterisk with a number from $a_1, a_2,\cdots, a_n$, while Rubi fills each asterisk with a number from $b_1, b_2,\cdots, b_n$, so that the equation on the whiteboard is correct. Suppose for every positive real number $t$, the number of ways for Ebi and Rubi to do so are equal.

Prove that the sequences $a_1,\cdots, a_n$ and $b_1, \cdots, b_n$ are permutations of each other.

(Note: $t=a_1a_2a_3$ and $t=a_2a_3a_1$ are considered different ways to fill the asterisks, and the chosen terms need not be distinct, for example $t=a_1a_1a_2$.)

Proposed by Wong Jer Ren
2 replies
navi_09220114
Yesterday at 1:01 PM
mashumaro
2 hours ago
Math Olympiad Workshops
kokcio   0
2 hours ago
Hello Math Enthusiasts!

I'm excited to announce a series of free Math Olympiad Workshops designed to help you sharpen your problem-solving skills in preparation for competitions. Whether you're a beginner or a seasoned competitor, these workshops aim to provide a supportive, challenging, and collaborative environment to explore advanced math topics.

Workshop Overview

Duration: 6 months (with the possibility of extending based on participant interest)

Structure: Weekly cycles, each dedicated to one of the main areas of Math Olympiad:
Week 1: Number Theory
Week 2: Geometry
Week 3: Algebra
Week 4: Combinatorics

Weekly Format
Monday: Problem Set Release: Approximately 30 problems will be posted covering the week's topic, which you will have chance to discuss.
Throughout the Week:
Theory Notes: I will share helpful theory and insights relevant to the problem set, giving you the tools you need to approach the problems.
Submission Opportunity: You can work on the problems and submit your solutions. I’ll review your work and provide feedback.
End of the Week: Solutions Post: I’ll release detailed solutions to all problems from the problem set.
Leaderboard: For those interested, we can maintain a table tracking participants who solve the most problems during the week.

Cycle Finale – Mock Contest
At the end of each 4-week cycle, we’ll host a Mock Contest featuring 4 problems (one from each topic). This is a great chance to simulate the competition environment and test your skills in a timed setting. I will review and provide feedback on your contest submissions.

Starting date: June 2

How to participate? Just write /signup under this post.

I believe these workshops will provide a comprehensive, engaging, and collaborative way to tackle Math Olympiad problems. I'm looking forward to seeing your creativity and problem-solving prowess!
If you have any questions or suggestions, please leave a comment below.
0 replies
kokcio
2 hours ago
0 replies
three "old" circles and four concurrent lines
pohoatza   50
N 3 hours ago by Sanjana42
Source: IMO Shortlist 2006, Geometry 6, AIMO 2007, TST 3, P3
Circles $ w_{1}$ and $ w_{2}$ with centres $ O_{1}$ and $ O_{2}$ are externally tangent at point $ D$ and internally tangent to a circle $ w$ at points $ E$ and $ F$ respectively. Line $ t$ is the common tangent of $ w_{1}$ and $ w_{2}$ at $ D$. Let $ AB$ be the diameter of $ w$ perpendicular to $ t$, so that $ A, E, O_{1}$ are on the same side of $ t$. Prove that lines $ AO_{1}$, $ BO_{2}$, $ EF$ and $ t$ are concurrent.
50 replies
pohoatza
Jun 28, 2007
Sanjana42
3 hours ago
Inequality and function
srnjbr   5
N Yesterday at 8:50 AM by pco
Find all f:R--R such that for all x,y, yf(x)+f(y)>=f(xy)
5 replies
srnjbr
Friday at 4:26 PM
pco
Yesterday at 8:50 AM
Inequality and function
G H J
G H BBookmark kLocked kLocked NReply
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srnjbr
56 posts
#1
Y by
Find all f:R--R such that for all x,y, yf(x)+f(y)>=f(xy)
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ektorasmiliotis
86 posts
#2
Y by
solution: f(x)=0
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pco
23460 posts
#3
Y by
ektorasmiliotis wrote:
solution: f(x)=0
Plus a lot lot of others
You should post your proof
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pco
23460 posts
#4
Y by
srnjbr wrote:
Find all f:R--R such that for all x,y, yf(x)+f(y)>=f(xy)
Let $P(x,y)$ be the assertion $yf(x)+f(y)\ge f(xy)$

Let $x,y\ne 0$ :
Adding $P(\frac xy,y)$ and $P(\frac xy,-y)$, we get $f(y)+f(-y)\ge f(x)+f(-x)$
And so (swapping $x,y$) : $f(x)+f(-x)=c$ for some constant $c$ and $\forall x\ne 0$

$P(x,-y)$ $\implies$ $-yf(x)+c-f(y)\ge c-f(xy)$ and so $f(xy)\ge yf(x)+f(y)$
And so, comparing with $P(x,y)$ $f(xy)=yf(x)+f(y)$ $\forall x,y\ne 0$

Swapping there $x,y$ and subtracting : $(y-1)f(x)=(x-1)f(y)$ $\forall x,y\ne 0$
And so $f(x)=a(x-1)$ for some constant $a$ and $\forall x\ne 0$

Let then $x\ne 0$ : $P(0,x)$ $\implies$ $xf(0)+a(x-1)\ge f(0)$ $\implies$ $(x-1)(f(0)+a)\ge 0$ and so $f(0)=-a$

And so $\boxed{f(x)=a(x-1)\quad\forall x}$ which indeed fits, whatever is $a\in\mathbb R$
This post has been edited 1 time. Last edited by pco, Yesterday at 8:50 AM
Reason: Typo
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srnjbr
56 posts
#5 • 2 Y
Y by aidan0626, pco
f(0)=-a.
Thank you very much for answering
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pco
23460 posts
#6
Y by
srnjbr wrote:
f(0)=-a.
Indeed :), thanks
Edited
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