Happy Memorial Day! Please note that AoPS Online is closed May 24-26th.

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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
May 1, 2025
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
jlacosta
May 1, 2025
0 replies
pairs (m, n) such that a fractional expression is an integer
cielblue   2
N 6 minutes ago by cielblue
Find all pairs $(m,\ n)$ of positive integers such that $\frac{m^3-mn+1}{m^2+mn+2}$ is an integer.
2 replies
cielblue
Yesterday at 8:38 PM
cielblue
6 minutes ago
Sociable set of people
jgnr   23
N 15 minutes ago by quantam13
Source: RMM 2012 day 1 problem 1
Given a finite number of boys and girls, a sociable set of boys is a set of boys such that every girl knows at least one boy in that set; and a sociable set of girls is a set of girls such that every boy knows at least one girl in that set. Prove that the number of sociable sets of boys and the number of sociable sets of girls have the same parity. (Acquaintance is assumed to be mutual.)

(Poland) Marek Cygan
23 replies
jgnr
Mar 3, 2012
quantam13
15 minutes ago
diophantine equation
m4thbl3nd3r   0
19 minutes ago
Find all positive integers $n,k$ such that $$5^{2n+1}-5^n+1=k^2$$
0 replies
m4thbl3nd3r
19 minutes ago
0 replies
A geometry problem
Lttgeometry   1
N 34 minutes ago by Funcshun840
Triangle $ABC$ has two isogonal conjugate points $P$ and $Q$. The circle $(BPC)$ intersects circle $(AP)$ at $R \neq P$, and the circle $(BQC)$ intersects circle $(AQ)$ at $S\neq Q$. Prove that $R$ and $S$ are isogonal conjugates in triangle $ABC$.
Note: Circle $(AP)$ is the circle with diameter $AP$, Circle $(AQ)$ is the circle with diameter $AQ$.
1 reply
Lttgeometry
Today at 4:03 AM
Funcshun840
34 minutes ago
Linear algebra
Feynmann123   5
N an hour ago by Feynmann123
Hi everyone,

I was wondering whether when I tried to compute e^(2x2 matrix) and got the expansions of sinx and cosx with the method of discounting the constant junk whether it plays any significance. I am a UK student and none of this is in my School syllabus so I was just wondering…


5 replies
Feynmann123
Yesterday at 6:44 PM
Feynmann123
an hour ago
[PMO20 Qualis] I. 15
Shinfu   0
2 hours ago
Suppose that ${a_n}$ is a nonconstant arithmetic sequence such that $a_1 = 1$ and the terms $a_3, a_{15}, a_{24}$ form a geometric sequence in that order. Find the smallest index $n$ for which $a_n < 0$.
$\text{(a) } 50 \qquad\text{(b) } 51 \qquad\text{(c) } 52 \qquad\text{(d) } 53$

Answer confirmation
0 replies
Shinfu
2 hours ago
0 replies
Minimize
lgx57   6
N 3 hours ago by lgx57
Minimize $\sqrt{\cos^2 x+(2-\sin x)^2}+\dfrac{1}{2}\sqrt{(\sqrt 3-\cos x)^2+(\sin x+1)^2}$
6 replies
lgx57
Friday at 1:29 PM
lgx57
3 hours ago
Inequalities
sqing   0
3 hours ago
Let $ a,b> 0 , \frac{a}{b^2}+\frac{b}{a^2}+\frac{56}{(a+b)^2} \leq 16.$ Prove that
$$ab(a+b) \geq 2$$Let $ a,b> 0 ,\frac{1}{a^2}+\frac{1}{b^2}+\frac{28}{(a+b)^2} \leq 9.$ Prove that
$$ab(a+b) \geq 2$$
0 replies
sqing
3 hours ago
0 replies
[Mathira 2018 T7-C2] Inverse of exponential spam
aops-g5-gethsemanea2   1
N 4 hours ago by aops-g5-gethsemanea2
Find the inverse of the function $f(x)=\dfrac12\left(\dfrac{2^{-x}+2^x}{2^{-x}-2^x}+\dfrac{2^{-x}-2^x}{2^{-x}+2^x}\right)$.
1 reply
aops-g5-gethsemanea2
4 hours ago
aops-g5-gethsemanea2
4 hours ago
Inequalities
toanrathay   1
N 4 hours ago by sqing
Let $a,b,c$ be positive reals such that $1/a+1/b+1/c-9/4abc=3/4$, find $\min$ of $P=a^2+b^2+c^2$.
1 reply
toanrathay
4 hours ago
sqing
4 hours ago
[Sipnayan SHS] Written Round, Average, #4.6
LilKirb   5
N 4 hours ago by LilKirb
Define the function
\[f(n) = \sum_{k=0}^{n} \binom{n}{k} a_k, \quad n = 1, 2, 3, \ldots\]where
\[a_k = 
    \begin{cases}
        3^k, & \text{if } k \text{ is even}, \\
        0, & \text{if } k \text{ is odd}.
        \end{cases}
\]Find the remainder when \( f(10^9 + 2) \) is divided by \( 2^{20} + 1 \)
5 replies
LilKirb
Yesterday at 2:25 PM
LilKirb
4 hours ago
La hire's with only euclidian
Kscv   0
5 hours ago
How can I prove la hire's theorem with pure Euclidean geometry? Every single proof I see on the internet uses projective geometry which I have no idea of.

Can someone help me? I only need the proof for circles BTW.
0 replies
1 viewing
Kscv
5 hours ago
0 replies
Penchick Manga Club
LilKirb   1
N 5 hours ago by LilKirb
From $96$ distinct penguin chicks, $47$ are selected to form the Penchick Manga Club. Then, one of these $47$ members is assigned to be club president. Let $x$ be the number of ways to select the club and assign its president. Compute $x \pmod{97}.$

1 reply
LilKirb
5 hours ago
LilKirb
5 hours ago
Asymmetric Inequality in the Reals
MrHeccMcHecc   1
N 6 hours ago by sqing
Let $a,b,c$ be real numbers such that $\frac{1}{abc}+\frac 1a + \frac 1c = \frac 1b$. Given the maximum value of $$\frac{4}{a^2+1}+\frac{4}{b^2+1}+\frac{7}{c^2+1}$$is $\frac{p}{q}$ where $p,q$ are relatively prime positive integers, find $p+q$.
1 reply
MrHeccMcHecc
Today at 4:46 AM
sqing
6 hours ago
Number Theory
AnhQuang_67   3
N Apr 24, 2025 by GreekIdiot
Source: HSGSO 2024
Let $p$ be an odd prime number and a sequence $\{a_n\}_{n=1}^{+\infty}$ satisfy $$a_1=1, a_2=2$$and $$a_{n+2}=2\cdot a_{n+1}+3\cdot a_n, \forall n \geqslant 1$$Prove that always exists positive integer $k$ satisfying for all positive integers $n$, then $a_n \ne k \mod{p}$.

P/s: $\ne$ is "not congruence"
3 replies
AnhQuang_67
Apr 24, 2025
GreekIdiot
Apr 24, 2025
Number Theory
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G H BBookmark kLocked kLocked NReply
Source: HSGSO 2024
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AnhQuang_67
57 posts
#1
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Let $p$ be an odd prime number and a sequence $\{a_n\}_{n=1}^{+\infty}$ satisfy $$a_1=1, a_2=2$$and $$a_{n+2}=2\cdot a_{n+1}+3\cdot a_n, \forall n \geqslant 1$$Prove that always exists positive integer $k$ satisfying for all positive integers $n$, then $a_n \ne k \mod{p}$.

P/s: $\ne$ is "not congruence"
This post has been edited 1 time. Last edited by AnhQuang_67, Apr 24, 2025, 2:54 PM
Reason: sorry, my bad
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GreekIdiot
256 posts
#2
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$p$ is an even prime? Then $p=2$ thus $a_n=0,1 \: mod \: 2$
and since $a_1,a_2$ are not of the same parity there doesnt exist such $k$
I assume you mean 'odd' prime
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AnhQuang_67
57 posts
#3
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GreekIdiot wrote:
$p$ is an even prime? Then $p=2$ thus $a_n=0,1 \: mod \: 2$
and since $a_1,a_2$ are not of the same parity there doesnt exist such $k$
I assume you mean 'odd' prime
Oops, sorry about that. It is "odd"
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GreekIdiot
256 posts
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