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k a March Highlights and 2025 AoPS Online Class Information
jlacosta   0
Mar 2, 2025
March is the month for State MATHCOUNTS competitions! Kudos to everyone who participated in their local chapter competitions and best of luck to all going to State! Join us on March 11th for a Math Jam devoted to our favorite Chapter competition problems! Are you interested in training for MATHCOUNTS? Be sure to check out our AMC 8/MATHCOUNTS Basics and Advanced courses.

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Do you have plans this summer? There are so many options to fit your schedule and goals whether attending a summer camp or taking online classes, it can be a great break from the routine of the school year. Check out our summer courses at AoPS Online, or if you want a math or language arts class that doesn’t have homework, but is an enriching summer experience, our AoPS Virtual Campus summer camps may be just the ticket! We are expanding our locations for our AoPS Academies across the country with 15 locations so far and new campuses opening in Saratoga CA, Johns Creek GA, and the Upper West Side NY. Check out this page for summer camp information.

Be sure to mark your calendars for the following events:
[list][*]March 5th (Wednesday), 4:30pm PT/7:30pm ET, HCSSiM Math Jam 2025. Amber Verser, Assistant Director of the Hampshire College Summer Studies in Mathematics, will host an information session about HCSSiM, a summer program for high school students.
[*]March 6th (Thursday), 4:00pm PT/7:00pm ET, Free Webinar on Math Competitions from elementary through high school. Join us for an enlightening session that demystifies the world of math competitions and helps you make informed decisions about your contest journey.
[*]March 11th (Tuesday), 4:30pm PT/7:30pm ET, 2025 MATHCOUNTS Chapter Discussion MATH JAM. AoPS instructors will discuss some of their favorite problems from the MATHCOUNTS Chapter Competition. All are welcome!
[*]March 13th (Thursday), 4:00pm PT/7:00pm ET, Free Webinar about Summer Camps at the Virtual Campus. Transform your summer into an unforgettable learning adventure! From elementary through high school, we offer dynamic summer camps featuring topics in mathematics, language arts, and competition preparation - all designed to fit your schedule and ignite your passion for learning.[/list]
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0 replies
jlacosta
Mar 2, 2025
0 replies
IMO 2023 P2
799786   89
N 5 minutes ago by kaede_Arcadia
Source: IMO 2023 P2
Let $ABC$ be an acute-angled triangle with $AB < AC$. Let $\Omega$ be the circumcircle of $ABC$. Let $S$ be the midpoint of the arc $CB$ of $\Omega$ containing $A$. The perpendicular from $A$ to $BC$ meets $BS$ at $D$ and meets $\Omega$ again at $E \neq A$. The line through $D$ parallel to $BC$ meets line $BE$ at $L$. Denote the circumcircle of triangle $BDL$ by $\omega$. Let $\omega$ meet $\Omega$ again at $P \neq B$. Prove that the line tangent to $\omega$ at $P$ meets line $BS$ on the internal angle bisector of $\angle BAC$.
89 replies
799786
Jul 8, 2023
kaede_Arcadia
5 minutes ago
combinatorıc
o.k.oo   0
26 minutes ago
A total of 3300 handshakes were made at a party attended by 600 people. It was observed
that the total number of handshakes among any 300 people at the party is at least N. Find
the largest possible value for N.
0 replies
o.k.oo
26 minutes ago
0 replies
Problem 2, Olympic Revenge 2013
hvaz   66
N 36 minutes ago by MonkeyLuffy
Source: XII Olympic Revenge - 2013
Let $ABC$ to be an acute triangle. Also, let $K$ and $L$ to be the two intersections of the perpendicular from $B$ with respect to side $AC$ with the circle of diameter $AC$, with $K$ closer to $B$ than $L$. Analogously, $X$ and $Y$ are the two intersections of the perpendicular from $C$ with respect to side $AB$ with the circle of diamter $AB$, with $X$ closer to $C$ than $Y$. Prove that the intersection of $XL$ and $KY$ lies on $BC$.
66 replies
hvaz
Jan 26, 2013
MonkeyLuffy
36 minutes ago
Functional equation wrapped in f's
62861   35
N an hour ago by ihatemath123
Source: RMM 2019 Problem 5
Determine all functions $f: \mathbb{R} \to \mathbb{R}$ satisfying
\[f(x + yf(x)) + f(xy) = f(x) + f(2019y),\]for all real numbers $x$ and $y$.
35 replies
62861
Feb 24, 2019
ihatemath123
an hour ago
No more topics!
Set closed under lcm(a,b)/(b-a)
MarkBcc168   3
N Mar 10, 2022 by puntre
Source: 2021 Thailand MO P9
Let $S$ be a set of positive integers such that if $a$ and $b$ are elements of $S$ such that $a<b$, then $b-a$ divides the least common multiple of $a$ and $b$, and the quotient is an element of $S$. Prove that the cardinality of $S$ is less than or equal to $2$.
3 replies
MarkBcc168
Nov 29, 2021
puntre
Mar 10, 2022
Set closed under lcm(a,b)/(b-a)
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G H BBookmark kLocked kLocked NReply
Source: 2021 Thailand MO P9
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MarkBcc168
1593 posts
#1 • 1 Y
Y by HWenslawski
Let $S$ be a set of positive integers such that if $a$ and $b$ are elements of $S$ such that $a<b$, then $b-a$ divides the least common multiple of $a$ and $b$, and the quotient is an element of $S$. Prove that the cardinality of $S$ is less than or equal to $2$.
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Sprites
478 posts
#2 • 1 Y
Y by HWenslawski
MarkBcc168 wrote:
Let $S$ be a set of positive integers such that if $a$ and $b$ are elements of $S$ such that $a<b$, then $b-a$ divides the least common multiple of $a$ and $b$, and the quotient is an element of $S$. Prove that the cardinality of $S$ is less than or equal to $2$.


Call such a set "special".Moreover let $f(a,b)=\frac{\text{lcm}(a,b)}{a-b}$ where $a>b$
We claim that no special set can contain $3$ elements.
Suppose such an set $\mathcal{S}=\{a_1>a_2>a_3 \}$ exists.
WLOG $\gcd(a_1,a_2)=d>1$(the condition $d=1$ gives the pair $\{2,z_1,2z_2 \}$ with $z_1$ odd which doesn't work).
First of all note that this gives $a_1=\frac{k_1k_2}{k_1-k_2}$ with $a_1=dk_1,a_2=dk_2$.Note that $\gcd(a_3,d)=1$ and let $d_2=\gcd(a_1,k_2)$ and $d_3=\gcd(a_1,k_3)$.
Now note that $$a_2<f(a_1,a_3)=\frac{\frac{k_1k_2}{d_2(k_1-k_2)} \cdot k_1}{\frac{k_1^2-2k_1k_2}{k_1-k_2}}<a_1 \not\in \mathcal{S}$$so this holds.

Induction. finishes.
This post has been edited 3 times. Last edited by Sprites, Dec 7, 2021, 5:38 AM
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txc
12 posts
#3 • 2 Y
Y by mijail, Quidditch
Suppose such a set $S$ exists.
Lemma. For any $a<b,a,b\in S$, we have $\frac{b}{a}=1+\frac{1}{k_1}$ for some $k_1\in\mathbb{N}$, and $k_1(k_1+1)\in S$.

Let $d=\gcd(a,b), a=k_1d,b=k_2d$. We have $$\frac{\text{lcm}(a,b)}{b-a}=\frac{k_1k_2}{k_2-k_1}\in\mathbb{Z}.$$But since $\gcd(k_1,k_2)=1,\gcd(k_1k_2,k_2-k_1)=1$, we must have $k_2-k_1=1$, which gives the above lemma.

Since $\frac{b}{a}\leq 1+\frac{1}{1}=2$ for any $a<b,a,b\in S$, $S$ must be finite. Let the elements of $S$ be $a_1<a_2<\dots<a_n$ for some $n\geq 3$.

Case 1. $\frac{a_3}{a_2}=\frac{a_2}{a_1}=1+\frac{1}{k}$.
Then $\frac{a_3}{a_1}=\left(1+\frac{1}{k}\right)^2=1+\frac{2k+1}{k^2}=1+\frac{1}{l}$ for some $l\in\mathbb{N}$, so $2k+1\mid k^2$, a contradiction.

Case 2. $\frac{a_3}{a_2}\neq\frac{a_2}{a_1}$.
Then the $n$ numbers $\frac{a_3}{a_2},\frac{a_2}{a_1},\frac{a_3}{a_1},\frac{a_4}{a_1},\frac{a_5}{a_1},\dots,\frac{a_n}{a_1}=1+\frac{1}{k}$ are all distinct. By the lemma, each distinct value of $\frac{a_j}{a_i}$ requires a distinct number of the form $m(m+1)$ in $S$, so all $n$ elements are of the form $m(m+1)$.
In particular, $a_1=k(k+1)$ since $\frac{a_n}{a_1}$ is the largest among the $n$ fractions. But then $a_n=k(k+1)\left(1+\frac{1}{k}\right)=(k+1)^2$ cannot be of the form $m(m+1)$, a contradiction. $\Box$
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puntre
159 posts
#4 • 1 Y
Y by Quidditch
Can't do it during the competition and figures it out a couple of months later. :blush:
Assume for the sake of contradiction that $|S|\geq 3.$
Claim: $b-a|\text{lcm}[a,b]\implies \text{gcd}(a,b)=b-a$.
Proof: Let $b_1=b-a,$ and $a=da'$, $b_1=db_1'$ where $d\in\mathbb{N}$ and $\gcd(a,b_1)=1$. Then \[b-a|\frac{ab}{\gcd(a,b)}\implies d^2b_1'|d^2(a')^2\implies b_1=1\implies\gcd(a,b_1+a)=b_1\]Hence $\gcd(a,b)=b-a.$

Using the claim, we conclude that $a,b\in S\implies \frac{ab}{(a-b)^2}\in S$.
Let $m<n$ be two smallest elements of $S$. Clearly, $\frac{mn}{(m-n)^2}\in S$, which implies that \[\frac{(n)(\frac{mn}{(m-n)^2})}{(n-\frac{mn}{(m-n)^2})^2}\in S\implies A=\frac{m}{(n-m)^2+\frac{m^2}{(n-m)^2}-2m}\in S.\]But $A$ is smaller than $n$ leads to a contradiction. So we are done.
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