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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 9th (Webinar), 4:00pm PT/7:00pm ET, Learn about Video-based Summer Camps at the Virtual Campus
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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
Maximum number of nice subsets
FireBreathers   1
N a few seconds ago by FireBreathers
Given a set $M$ of natural numbers with $n$ elements with $n$ odd number. A nonempty subset $S$ of $M$ is called $nice$ if the product of the elements of $S$ divisible by the sum of the elements of $M$, but not by its square. It is known that the set $M$ itself is good. Determine the maximum number of $nice$ subsets (including $M$ itself).
1 reply
FireBreathers
Yesterday at 10:27 PM
FireBreathers
a few seconds ago
Existence of reals satisfying cyclic relation
DVDthe1st   10
N 12 minutes ago by sttsmet
Source: 2018 China TST Day 1 Q1
Let $p,q$ be positive reals with sum 1. Show that for any $n$-tuple of reals $(y_1,y_2,...,y_n)$, there exists an $n$-tuple of reals $(x_1,x_2,...,x_n)$ satisfying $$p\cdot \max\{x_i,x_{i+1}\} + q\cdot \min\{x_i,x_{i+1}\} = y_i$$for all $i=1,2,...,2017$, where $x_{2018}=x_1$.
10 replies
DVDthe1st
Jan 2, 2018
sttsmet
12 minutes ago
Inspired by 2024 Fall LMT Guts
sqing   1
N 19 minutes ago by sqing
Source: Own
Let $x$, $y$, $z$ are pairwise distinct real numbers satisfying $x^2+y =y^2 +z = z^2+x. $ Prove that
$$(x+y)(y+z)(z+x)=-1$$Let $x$, $y$, $z$ are pairwise distinct real numbers satisfying $x^2+2y =y^2 +2z = z^2+2x. $ Prove that
$$(x+y)(y+z)(z+x)=-8$$
1 reply
sqing
25 minutes ago
sqing
19 minutes ago
How many non-attacking pawns can be placed on a $n \times n$ chessboard?
DylanN   2
N 21 minutes ago by zRevenant
Source: 2019 Pan-African Shortlist - C1
A pawn is a chess piece which attacks the two squares diagonally in front if it. What is the maximum number of pawns which can be placed on an $n \times n$ chessboard such that no two pawns attack each other?
2 replies
DylanN
Jan 18, 2021
zRevenant
21 minutes ago
circumcenter, excenter and vertex collinear (Singapore Junior 2012)
parmenides51   6
N Today at 6:24 AM by lightsynth123
In $\vartriangle ABC$, the external bisectors of $\angle A$ and $\angle B$ meet at a point $D$. Prove that the circumcentre of $\vartriangle ABD$ and the points $C, D$ lie on the same straight line.
6 replies
parmenides51
Jul 11, 2019
lightsynth123
Today at 6:24 AM
can anyone solve this
averageguy   9
N Today at 6:18 AM by ninjaforce
Hi guys,
For some reason I can't think of a simple way to solve this problem. Is there anyway you guys can think of without trig or if it does have trig something elegant. Answer is 106 btw.
9 replies
averageguy
Dec 26, 2024
ninjaforce
Today at 6:18 AM
Inequalities
sqing   5
N Today at 4:00 AM by sqing
Let $ a,b,c $ be real numbers such that $ a^2+b^2+c^2=1. $ Prove that$$ |a-b|+|b-2c|+|c-3a|\leq 5$$$$|a-2b|+|b-3c|+|c-4a|\leq \sqrt{42}$$$$ |a-b|+|b-\frac{11}{10}c|+|c-a|\leq \frac{29}{10}$$
5 replies
sqing
Apr 22, 2025
sqing
Today at 4:00 AM
Complex Numbers Question
franklin2013   3
N Today at 3:47 AM by KSH31415
Hello everyone! This is one of my favorite complex numbers questions. Have fun!

$f(z)=z^{720}-z^{120}$. How many complex numbers $z$ are there such that $|z|=1$ and $f(z)$ is an integer.

Hint
3 replies
franklin2013
Apr 20, 2025
KSH31415
Today at 3:47 AM
Inequalities
sqing   28
N Today at 2:53 AM by sqing
Let $   a,b    $ be reals such that $  a^2+ab+b^2 =3$ . Prove that
$$ \frac{4}{ 3}\geq \frac{1}{ a^2+5 }+ \frac{1}{ b^2+5 }+ab \geq -\frac{11}{4 }$$$$ \frac{13}{ 4}\geq \frac{1}{ a^2+5 }+ \frac{1}{ b^2+5 }+ab \geq -\frac{2}{3 }$$$$ \frac{3}{ 2}\geq  \frac{1}{ a^4+3 }+ \frac{1}{ b^4+3 }+ab \geq -\frac{17}{6 }$$$$ \frac{19}{ 6}\geq  \frac{1}{ a^4+3 }+ \frac{1}{ b^4+3 }-ab \geq -\frac{1}{2}$$Let $   a,b    $ be reals such that $  a^2-ab+b^2 =1 $ . Prove that
$$ \frac{3}{ 2}\geq \frac{1}{ a^2+3 }+ \frac{1}{ b^2+3 }+ab \geq \frac{4}{15 }$$$$ \frac{14}{ 15}\geq \frac{1}{ a^2+3 }+ \frac{1}{ b^2+3 }-ab \geq -\frac{1}{2 }$$$$ \frac{3}{ 2}\geq \frac{1}{ a^4+3 }+ \frac{1}{ b^4+3 }+ab \geq \frac{13}{42 }$$$$ \frac{41}{ 42}\geq \frac{1}{ a^4+3 }+ \frac{1}{ b^4+3 }-ab \geq -\frac{1}{2 }$$
28 replies
sqing
Apr 16, 2025
sqing
Today at 2:53 AM
Middle School Math <3
peace09   13
N Yesterday at 11:39 PM by Kevin2010
If $f(0)=1$ and $f(n)=\tfrac{n!}{\text{lcm}(1,2,\dots,n)}$ for each positive integer $n$, what is the value of $\tfrac{f(1)}{f(0)}+\tfrac{f(2)}{f(1)}+\dots+\tfrac{f(50)}{f(49)}$?

If you enjoyed the above problem, check out the 2024 WMC Series!
13 replies
peace09
Mar 11, 2024
Kevin2010
Yesterday at 11:39 PM
Theory of Equations
P162008   2
N Yesterday at 10:18 PM by P162008
Let $a,b,c,d$ and $e\in [-2,2]$ such that $\sum_{cyc} a = 0, \sum_{cyc} a^3 = 0, \sum_{cyc} a^5 = 10.$ Find the value of $\sum_{cyc} a^2.$
2 replies
P162008
Yesterday at 11:27 AM
P162008
Yesterday at 10:18 PM
inequalities of elements in set
toanrathay   1
N Yesterday at 7:25 PM by Lankou
Let \( m \) be a positive integer such that \( m \geq 4 \), and let the set
\[
A = \{a_1, a_2, a_3, \ldots, a_m\}
\]consist of distinct positive integers not exceeding 2025. Suppose that for every \( a, b \in A \), with \( a \ne b \), if \( a + b \leq 2025 \), then \( a + b \in A \) as well. Prove that

\[
\frac{a_1 + a_2 + a_3 + \cdots + a_m}{m} \geq 1013.
\]
1 reply
toanrathay
Yesterday at 3:33 PM
Lankou
Yesterday at 7:25 PM
2024 PUMaC Team Round, Question 14 Inquiry
A22-4   0
Yesterday at 6:39 PM
2024 PUMaC Team Round Question 14 reads as follows:

What is the largest value of $m$ for which I can find nonnegative integers $a_1, a_2, \cdots, a_m<2024$ such that for all indices $i>j$, $17$ divides $\binom{a_i}{a_j}$?
(Note: This should say "... nonnegative integers $a_1<a_2<\cdots<a_m<2024$ ...")

I interpreted this correction to mean the following:
What is the largest value of $m$ for which I there exists nonnegative integers $a_1<a_2<\cdots<a_m<2024$ such that for all indices $i>j$, $17$ divides $\binom{a_i}{a_j}$?

The official answer (https://static1.squarespace.com/static/570450471d07c094a39efaed/t/67421bd74806e80a7ab11c7d/1732385751115/PUMaC_2024_Team__Final_.pdf) is 107. However, I believe I have a construction with $108$ integers - take the set of all integers with a digit sum of $19$ in base $17$, then append $2023_{10}=700_{17}$ to the list.

I checked this with Python using the following code:
[code]def digit_sum_base(n, base):
total = 0
while n > 0:
total += n % base
n //= base
return total

target_sum = 19
base = 17
limit = 2024
qualified_numbers = [n for n in range(limit) if digit_sum_base(n, base) == target_sum]

qualified_numbers.append(2023)

from math import comb

all_divisible = True
for i in range(len(qualified_numbers)):
for j in range(i):
a, b = qualified_numbers, qualified_numbers[j]
if comb(a, b) % 17 != 0:
all_divisible = False
break
if not all_divisible:
break

print(len(qualified_numbers), all_divisible)[/code]

Am I wrong or are they wrong? Any insight would be appreciated!
0 replies
A22-4
Yesterday at 6:39 PM
0 replies
How many ways can we indistribute n different marbles into 6 identical boxes
Taiharward   10
N Yesterday at 5:33 PM by MathBot101101
How many ways can we distribute n indifferent marbles into 6 identical boxes and one jar?
10 replies
Taiharward
Yesterday at 2:14 AM
MathBot101101
Yesterday at 5:33 PM
Containment in a circle
Zelderis   2
N Mar 14, 2024 by bin_sherlo
Source: Brazil National Olympiad 2019 - level 2 - #4
Let $ ABC $ be an acutangle triangle and $ D $ any point on the $ BC $ side. Let $ E $ be the symmetrical of $ D $ in
$ AC $ and $ F $ is the symmetrical $ D $ relative to $ AB $. $ A $ straight $ ED $ intersects straight $ AB $ at $ G $, while straight $ F D $ intersects the line $ AC $ in $ H $. Prove that the points $ A, E, F, G$ and $ H $ are on the same circumference.
2 replies
Zelderis
Nov 23, 2019
bin_sherlo
Mar 14, 2024
Containment in a circle
G H J
Source: Brazil National Olympiad 2019 - level 2 - #4
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Zelderis
339 posts
#1 • 1 Y
Y by Adventure10
Let $ ABC $ be an acutangle triangle and $ D $ any point on the $ BC $ side. Let $ E $ be the symmetrical of $ D $ in
$ AC $ and $ F $ is the symmetrical $ D $ relative to $ AB $. $ A $ straight $ ED $ intersects straight $ AB $ at $ G $, while straight $ F D $ intersects the line $ AC $ in $ H $. Prove that the points $ A, E, F, G$ and $ H $ are on the same circumference.
This post has been edited 1 time. Last edited by Zelderis, Nov 23, 2019, 1:16 PM
Z K Y
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Pluto1708
1107 posts
#2 • 5 Y
Y by amar_04, AlastorMoody, Smkh, A-Thought-Of-God, Adventure10
Cmon this is obvious
Z K Y
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bin_sherlo
708 posts
#3
Y by
Above is true. But I won't chase angle
$AB\cap EF=K,AC\cap FE=L,AB\cap FD=M,AC\cap ED=N$
$\angle KDA=\angle MDA-\angle MDK=90-\angle DAM-\angle KFD=90-\angle DAM-\angle NMD=90-\angle A=\angle AGD$
gives that $AD^2=AK.AG$ Similarily $AD^2=AL.AH$
Inversion centered at $A$ with radius $AD=AE=AF$ gives that $FKLE\rightarrow (AFGHE)$ as desired.
Z K Y
N Quick Reply
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