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k a July Highlights and 2025 AoPS Online Class Information
jwelsh   0
Jul 1, 2025
We are halfway through summer, so be sure to carve out some time to keep your skills sharp and explore challenging topics at AoPS Online and our AoPS Academies (including the Virtual Campus)!

[list][*]Over 60 summer classes are starting at the Virtual Campus on July 7th - check out the math and language arts options for middle through high school levels.
[*]At AoPS Online, we have accelerated sections where you can complete a course in half the time by meeting twice/week instead of once/week, starting on July 8th:
[list][*]MATHCOUNTS/AMC 8 Basics
[*]MATHCOUNTS/AMC 8 Advanced
[*]AMC Problem Series[/list]
[*]Plus, AoPS Online has a special seminar July 14 - 17 that is outside the standard fare: Paradoxes and Infinity
[*]We are expanding our in-person AoPS Academy locations - are you looking for a strong community of problem solvers, exemplary instruction, and math and language arts options? Look to see if we have a location near you and enroll in summer camps or academic year classes today! New locations include campuses in California, Georgia, New York, Illinois, and Oregon and more coming soon![/list]

MOP (Math Olympiad Summer Program) just ended and the IMO (International Mathematical Olympiad) is right around the corner! This year’s IMO will be held in Australia, July 10th - 20th. Congratulations to all the MOP students for reaching this incredible level and best of luck to all selected to represent their countries at this year’s IMO! Did you know that, in the last 10 years, 59 USA International Math Olympiad team members have medaled and have taken over 360 AoPS Online courses. Take advantage of our Worldwide Online Olympiad Training (WOOT) courses
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0 replies
jwelsh
Jul 1, 2025
0 replies
diophantine with factorials and exponents
skellyrah   16
N 9 minutes ago by maromex
find all positive integers $a,b,c$ such that $$ a! + 5^b = c^3 $$
16 replies
skellyrah
May 30, 2025
maromex
9 minutes ago
australian amc 2010 hardest question ask for help
zhufengnew   0
13 minutes ago
Source: AMC 2010 question 30
There are many towns on the island of Tetra, all connected by roads. Each town
has three roads leading to three other different towns: one red road, one yellow
road and one blue road, where no two roads meet other than at towns. If you
start from any town and travel along red and yellow roads alternately (RYRY ... )
you will get back to your starting town after having travelled over six different
roads. In fact RYRYRY will always get you back to where you started. In the��
same way, going along yellow and blue roads alternately will always get you back
to the starting point after travelling along six different roads (YBYBYB). On the
other hand, gol.ng along red and blue roads alternately will always get you back to
the starting point after travelling along four different roads (RBRB). How many
towns are there on Tetra?

I have read the official solution, it 24. but it is not clear what's the principle can guide to solve this problem. can any master share with his/her strategy on solving it? thanks so much.
0 replies
+1 w
zhufengnew
13 minutes ago
0 replies
easy modified cauchy fe
blueprimes   6
N 15 minutes ago by AshAuktober
Source: OTIS Z2A3B740
Determine all functions $f:\mathbb{R}_{\ge 0} \to \mathbb{R}_{\ge 0}$ such that
\[ f(x) + f(y) + 2xy = f(x + y) \]for all nonnegative real numbers $x$ and $y$.
6 replies
+1 w
blueprimes
Feb 24, 2025
AshAuktober
15 minutes ago
Convex quad
MithsApprentice   84
N 23 minutes ago by Shan3t
Source: USAMO 1993
Let $\, ABCD \,$ be a convex quadrilateral such that diagonals $\, AC \,$ and $\, BD \,$ intersect at right angles, and let $\, E \,$ be their intersection. Prove that the reflections of $\, E \,$ across $\, AB, \, BC, \, CD, \, DA \,$ are concyclic.
84 replies
MithsApprentice
Oct 27, 2005
Shan3t
23 minutes ago
Weird parity (idk maybe) problem
Ro.Is.Te.   0
an hour ago
Given the equation:
$\frac{1}{x - y - z} = \frac{1}{y} + \frac{1}{z}$
How many ordered triples $(x,y,z)$ are either prime numbers or the negatives of prime numbers?
0 replies
Ro.Is.Te.
an hour ago
0 replies
Sum of recurrence
Kyj9981   1
N an hour ago by Kyj9981
source: Sipnayan SHS Elims 2018/V1

Let $s_0=6$, $s_1=6$, and $s_n=2s_{n-1}+8s_{n-2}$ for $n \geq 2$. Define
\[A_n=\sum_{i=0}^n s_{i}\]Find $A_{2018}$. Express your answer in the form $a^b+c^d$, where $a$, $b$, $c$, and $d$ are positive integers.
1 reply
Kyj9981
2 hours ago
Kyj9981
an hour ago
[PMO27 Areas] I.13 are you sure
BinariouslyRandom   4
N 3 hours ago by Kyj9981
The sequence of real numbers $x_1, x_2, \dots$, satisfies the recurrence relation
\[ \frac{x_{n+1}}{x_n} = \frac{(x_{n+1})^2 + 27}{x_n^2 + 27} \]for all positive integers $n$. Suppose that $x_{20} = x_{25} = 3$. Let $M$ be the maximum value of
\[ \sum_{n=1}^{2025} x_n. \]What is $M \pmod{1000}$?
4 replies
BinariouslyRandom
Jan 25, 2025
Kyj9981
3 hours ago
Challenge: Make as many positive integers from 2 zeros
Biglion   25
N 3 hours ago by littleduckysteve
How many positive integers can you make from at most 2 zeros, any math operation and cocatination?
New Rule: The successor function can only be used at most 3 times per number
Starting from 0, 0=0
25 replies
Biglion
Jul 2, 2025
littleduckysteve
3 hours ago
10 Problems
Sedro   9
N 3 hours ago by fruitmonster97
Title says most of it. I've been meaning to post a problem set on HSM since at least a few months ago, but since I proposed the most recent problems I made to the 2025 SSMO, I had to wait for that happen. (Hence, most of these problems will probably be familiar if you participated in that contest, though numbers and wording may be changed.) The problems are very roughly arranged by difficulty. Enjoy!

Problem 1: An increasing sequence of positive integers $u_1, u_2, \dots, u_8$ has the property that the sum of its first $n$ terms is divisible by $n$ for every positive integer $n\le 8$. Let $S$ be the number of such sequences satisfying $u_1+u_2+\cdots + u_8 = 144$. Compute the remainder when $S$ is divided by $1000$.

Problem 2: Rhombus $PQRS$ has side length $3$. Point $X$ lies on segment $PR$ such that line $QX$ is perpendicular to line $PS$. Given that $QX=2$, the area of $PQRS$ can be expressed as $\tfrac{m}{n}$, where $m$ and $n$ are relatively prime positive integers. Compute $m+n$.

Problem 3: Positive integers $a$ and $b$ satisfy $a\mid b^2$, $b\mid a^3$, and $a^3b^2 \mid 2025^{36}$. If the number of possible ordered pairs $(a,b)$ is equal to $N$, compute the remainder when $N$ is divided by $1000$.

Problem 4: Let $ABC$ be a triangle. Point $P$ lies on side $BC$, point $Q$ lies on side $AB$, and point $R$ lies on side $AC$ such that $PQ=BQ$, $CR=PR$, and $\angle APB<90^\circ$. Let $H$ be the foot of the altitude from $A$ to $BC$. Given that $BP=3$, $CP=5$, and $[AQPR] = \tfrac{3}{7} \cdot [ABC]$, the value of $BH\cdot CH$ can be expressed as $\tfrac{m}{n}$, where $m$ and $n$ are relatively prime positive integers. Compute $m+n$.

Problem 5: Anna has a three-term arithmetic sequence of integers. She divides each term of her sequence by a positive integer $n>1$ and tells Bob that the three resulting remainders are $20$, $52$, and $R$, in some order. For how many values of $R$ is it possible for Bob to uniquely determine $n$?

Problem 6: There is a unique ordered triple of positive reals $(x,y,z)$ satisfying the system of equations \begin{align*} x^2 + 9 &= (y-\sqrt{192})^2 + 4 \\ y^2 + 4 &= (z-\sqrt{192})^2 + 49 \\ z^2 + 49 &= (x-\sqrt{192})^2 + 9. \end{align*}The value of $100x+10y+z$ can be expressed as $p\sqrt{q}$, where $p$ and $q$ are positive integers such that $q$ is square-free. Compute $p+q$.

Problem 7: Let $S$ be the set of all monotonically increasing six-term sequences whose terms are all integers between $0$ and $6$ inclusive. We say a sequence $s=n_1, n_2, \dots, n_6$ in $S$ is symmetric if for every integer $1\le i \le 6$, the number of terms of $s$ that are at least $i$ is $n_{7-i}$. The probability that a randomly chosen element of $S$ is symmetric is $\tfrac{p}{q}$, where $p$ and $q$ are relatively prime positive integers. Compute $p+q$.

Problem 8: For a positive integer $n$, let $r(n)$ denote the value of the binary number obtained by reading the binary representation of $n$ from right to left. Find the smallest positive integer $k$ such that the equation $n+r(n)=2k$ has at least ten positive integer solutions $n$.

Problem 9: Let $p$ be a quadratic polynomial with a positive leading coefficient. There exists a positive real number $r$ such that $r < 1 < \tfrac{5}{2r} < 5$ and $p(p(x)) = x$ for $x \in \{ r,1,  \tfrac{5}{2r} , 5\}$. Compute $p(20)$.

Problem 10: Find the number of ordered triples of positive integers $(a,b,c)$ such that $a+b+c=995$ and $ab+bc+ca$ is a multiple of $995$.
9 replies
Sedro
Jul 10, 2025
fruitmonster97
3 hours ago
[PMO26 Qualifying II.12] Equality
kae_3   5
N 4 hours ago by fruitmonster97
The real numbers $x,y$ are such that $x\neq y$ and \[\frac{x}{26-x^2}=\frac{y}{26-y^2}=\frac{xy}{26-(xy)^2}.\]What is $x^2+y^2$?

$\text{(a) }626\qquad\text{(b) }650\qquad\text{(c) }677\qquad\text{(d) }729$

Answer Confirmation
5 replies
kae_3
Feb 21, 2025
fruitmonster97
4 hours ago
Trigonometry equation practice
ehz2701   8
N 5 hours ago by vanstraelen
There is a lack of trigonometric bash practice, and I want to see techniques to do these problems. So here are 10 kinds of problems that are usually out in the wild. How do you tackle these problems? (I had ChatGPT write a code for this.). Please give me some general techniques to solve these kinds of problems, especially set 2b. I’ll add more later.

Leaderboard

problem set 1a

problem set 2a

problem set 2b
answers 2b

General techniques so far:

Trick 1: one thing to keep in mind is

[center] $\frac{1}{2} = \cos 36 - \sin 18$. [/center]

Many of these seem to be reducible to this. The half can be written as $\cos 60 = \sin 30$, and $\cos 36 = \sin 54$, $\sin 18 = \cos 72$. This is proven in solution 1a-1. We will refer to this as Trick 1.
8 replies
ehz2701
Jul 12, 2025
vanstraelen
5 hours ago
Permutations of n-degree
red_dog   1
N 5 hours ago by alexheinis
Let $S_n$ be the set of permutations of n-degree and $\sigma\in S_n, \ n\ge 3$. Prove that if $\sigma\alpha=\alpha\sigma, \ \forall \alpha\in S_n$, then $\sigma=e$ (where $e$ is the identical permutation).
1 reply
red_dog
6 hours ago
alexheinis
5 hours ago
Subgroups
red_dog   1
N 5 hours ago by alexheinis
Let $G_1,G_2,\ldots,G_{2002}$ be subgroups of the group $(\mathbb{Q},+)$ and $\mathbb{Q}=G_1\cup G_2\cup\ldots\cup G_{2002}$. Prove that exists $i\in\{1,2,\ldots,2002\}$ such as $G_i=\mathbb{Q}$.
1 reply
red_dog
6 hours ago
alexheinis
5 hours ago
Modular Equality with Coprime Integers
pandev3   5
N Today at 7:10 AM by pandev3
Let $(a)_n = a - n \left\lfloor \frac{a}{n} \right\rfloor$, which is equivalent to $a \mod n$ for any integer $a$.

Let $a_1, a_2, a_3, a_4$ and $n$ be positive integers such that $a_i$ is coprime with $n$ for $i = 1, 2, 3, 4$.

It holds that $(k a_1)_n + (k a_2)_n + (k a_3)_n + (k a_4)_n = 2n$ for $k = 1, 2, \dots, n - 1$.

Prove that $(a_1)_n + (a_j)_n = n$ for some $j$ where $2 \leq j \leq 4$.
5 replies
pandev3
Nov 21, 2024
pandev3
Today at 7:10 AM
Difficult combinatorics problem
shactal   7
N May 19, 2025 by shactal
Can someone help me with this problem? Let $n\in \mathbb N^*$. We call a distribution the act of distributing the integers from $1$
to $n^2$ represented by tokens to players $A_1$ to $A_n$ so that they all have the same number of tokens in their urns.
We say that $A_i$ beats $A_j$ when, when $A_i$ and $A_j$ each draw a token from their urn, $A_i$ has a strictly greater chance of drawing a larger number than $A_j$. We then denote $A_i>A_j$. A distribution is said to be chicken-fox-viper when $A_1>A_2>\ldots>A_n>A_1$ What is $R(n)$
, the number of chicken-fox-viper distributions?
7 replies
shactal
May 18, 2025
shactal
May 19, 2025
Difficult combinatorics problem
G H J
G H BBookmark kLocked kLocked NReply
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shactal
9 posts
#1
Y by
Can someone help me with this problem? Let $n\in \mathbb N^*$. We call a distribution the act of distributing the integers from $1$
to $n^2$ represented by tokens to players $A_1$ to $A_n$ so that they all have the same number of tokens in their urns.
We say that $A_i$ beats $A_j$ when, when $A_i$ and $A_j$ each draw a token from their urn, $A_i$ has a strictly greater chance of drawing a larger number than $A_j$. We then denote $A_i>A_j$. A distribution is said to be chicken-fox-viper when $A_1>A_2>\ldots>A_n>A_1$ What is $R(n)$
, the number of chicken-fox-viper distributions?
Z K Y
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shactal
9 posts
#2
Y by
Someone?
Z K Y
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aaravdodhia
2667 posts
#3
Y by
Isn't it $0$?

Note that $A_i$ beats $A_j$ when $E(d_i-d_j)>0$, where $E$ represents expected value and $d_i, d_j$ are the draws of players $i$ and $j$. That happens when $E(d_i) - E(d_j)>0$, or the sum of $A_i$'s collection is greater than the sum of $A_j$'s collection. In the problem, the sum of $A_1$'s collection must be greater than the sum of everybody else's, contradicting $A_n > A_1$.

This logic is due to the distribution being given prior to the players drawing and comparing their numbers. But if all distributions were considered at once, any pair $(i,j)$ would satisfy $A_i > A_j$ with equal probability $\tfrac12\left(\text{probability }E(d_i-d_j)\ne0\right)$, so the player's expected draws would all be the same. Hence there cannot be an order to $A_1\dots A_n$.

If this problem is from another source, I'd suggest reading their explanation. :)
Z K Y
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shactal
9 posts
#4
Y by
Well, the thing is I don't have the solution and I would like to know the method to solve this type of problems
Z K Y
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Ash_the_Bash07
1332 posts
#5
Y by
ok$~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~$
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shactal
9 posts
#6
Y by
But I don't think the answer is $0$, because I already found some examples where the condition is satisfied
Z K Y
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shactal
9 posts
#7
Y by
Here is an example that satisfies the condition: Player $A$ has numbers $\{2,4,9\}$, player B has $\{1,6,8\}$ and player $C$ has $\{3,5,7\}$
Z K Y
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shactal
9 posts
#8
Y by
If I can show that the events "$A$ wins against $B$" and "$B$ wins against $C$" are independent, then the problem is trivial. But how to prove this?
Z K Y
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