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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
easy number theory sequnce problem
skellyrah   2
N 8 minutes ago by skellyrah
Source: simillar to 2016 Greece,Team Selection Test,Problem
Define the sequnce ${(a_n)}_{n\ge0}$ by $a_0=3$ and $a_n=2a_{n-1}+1$
Determine all positive integers $m$ such that $\gcd (m,a_n)=1 \ , \ \forall n\geq 0$.
2 replies
skellyrah
32 minutes ago
skellyrah
8 minutes ago
Finding a subsquare from the main square
goodar2006   2
N 15 minutes ago by quantam13
Source: Iran 3rd round 2012-Special Lesson exam-Part 2-P4
Prove that if $n$ is large enough, in every $n\times n$ square that a natural number is written on each one of its cells, one can find a subsquare from the main square such that the sum of the numbers is this subsquare is divisible by $1391$.
2 replies
goodar2006
Sep 15, 2012
quantam13
15 minutes ago
Three sets having the same color
goodar2006   2
N 16 minutes ago by quantam13
Source: Iran 3rd round 2012-Special Lesson exam-Part 2-P3
Prove that if $n$ is large enough, then for each coloring of the subsets of the set $\{1,2,...,n\}$ with $1391$ colors, two non-empty disjoint subsets $A$ and $B$ exist such that $A$, $B$ and $A\cup B$ are of the same color.
2 replies
goodar2006
Sep 15, 2012
quantam13
16 minutes ago
1000 points with distinct pairwise distances
goodar2006   2
N 20 minutes ago by quantam13
Source: Iran 3rd round 2012-Special Lesson exam-Part1-P3
Prove that if $n$ is large enough, among any $n$ points of plane we can find $1000$ points such that these $1000$ points have pairwise distinct distances. Can you prove the assertion for $n^{\alpha}$ where $\alpha$ is a positive real number instead of $1000$?
2 replies
goodar2006
Jul 27, 2012
quantam13
20 minutes ago
rare creative geo problem spotted in the wild
abbominable_sn0wman   4
N an hour ago by abbominable_sn0wman
The following is the construction of the twindragon fractal.

Let $I_0$ be the solid square region with vertices at
\[
(0, 0), \left(\frac{1}{2}, \frac{1}{2}\right), (1, 0), \left(\frac{1}{2}, -\frac{1}{2}\right).
\]
Recursively, the region $I_{n+1}$ consists of two copies of $I_n$: one copy which is rotated $45^\circ$ counterclockwise around the origin and scaled by a factor of $\frac{1}{\sqrt{2}}$, and another copy which is also rotated $45^\circ$ counterclockwise around the origin and scaled by a factor of $\frac{1}{\sqrt{2}}$, and then translated by $\left(\frac{1}{2}, -\frac{1}{2}\right)$.

We have displayed $I_0$ and $I_1$ below.

Let $I_\infty$ be the limiting region of the sequence $I_0, I_1, \dots$.

The area of the smallest convex polygon which encloses $I_\infty$ can be written as $\frac{a}{b}$ for relatively prime positive integers $a$ and $b$. Find $a + b$.
4 replies
abbominable_sn0wman
Yesterday at 6:04 PM
abbominable_sn0wman
an hour ago
Indonesia Juniors 2012 day 2 OSN SMP
parmenides51   3
N an hour ago by Rayholr123
p1. One day, a researcher placed two groups of species that were different, namely amoeba and bacteria in the same medium, each in a certain amount (in unit cells). The researcher observed that on the next day, which is the second day, it turns out that every cell species divide into two cells. On the same day every cell amoeba prey on exactly one bacterial cell. The next observation carried out every day shows the same pattern, that is, each cell species divides into two cells and then each cell amoeba prey on exactly one bacterial cell. Observation on day $100$ shows that after each species divides and then each amoeba cell preys on exactly one bacterial cell, it turns out kill bacteria. Determine the ratio of the number of amoeba to the number of bacteria on the first day.


p2. It is known that $n$ is a positive integer. Let $f(n)=\frac{4n+\sqrt{4n^2-1}}{\sqrt{2n+1}+\sqrt{2n-1}}$.
Find $f(13) + f(14) + f(15) + ...+ f(112).$


p3. Budi arranges fourteen balls, each with a radius of $10$ cm. The first nine balls are placed on the table so that
form a square and touch each other. The next four balls placed on top of the first nine balls so that they touch each other. The fourteenth ball is placed on top of the four balls, so that it touches the four balls. If Bambang has fifty five balls each also has a radius of $10$ cm and all the balls are arranged following the pattern of the arrangement of the balls made by Budi, calculate the height of the center of the topmost ball is measured from the table surface in the arrangement of the balls done by Bambang.


p4. Given a triangle $ABC$ whose sides are $5$ cm, $ 8$ cm, and $\sqrt{41}$ cm. Find the maximum possible area of the rectangle can be made in the triangle $ABC$.


p5. There are $12$ people waiting in line to buy tickets to a show with the price of one ticket is $5,000.00$ Rp.. Known $5$ of them they only have $10,000$ Rp. in banknotes and the rest is only has a banknote of $5,000.00$ Rp. If the ticket seller initially only has $5,000.00$ Rp., what is the probability that the ticket seller have enough change to serve everyone according to their order in the queue?
3 replies
parmenides51
Nov 3, 2021
Rayholr123
an hour ago
2018 Sipnayan Junior Highscool Semifinals A Average.1
wonderboy807   0
an hour ago
Let f(1) = 2016 , f(2) = 2018 ,

f(n) = [f(n-1)]^2 + [f(n-2)]^2 \quad \text{for all } n \geq 3.

What is the units digit of f(2018) ?

Answer: Click to reveal hidden text

Solution: Click to reveal hidden text
0 replies
wonderboy807
an hour ago
0 replies
PMO 2020 Qualifying Stage I.6
wonderboy807   0
2 hours ago
A function f : \mathbb{R} \to \mathbb{R} satisfies f(xy) = \frac{f(x)}{y^2} for all positive real numbers x and y . Given that f(25) = 48, what is f(100) ?

Answer: Click to reveal hidden text

Solution: Click to reveal hidden text
0 replies
wonderboy807
2 hours ago
0 replies
Indonesian Junior MO (Nationals) 2018, Day 2
somebodyyouusedtoknow   1
N 2 hours ago by Rayholr123
P6. It is given the integer $Y$ with
$Y = 2018 + 20118 + 201018 + 2010018 + \cdots + 201 \underbrace{00 \ldots 0}_{\textrm{100 digits}} 18.$
Determine the sum of all the digits of such $Y$. (It is implied that $Y$ is written with a decimal representation.)

P7. Three groups of lines divides a plane into $D$ regions. Every pair of lines in the same group are parallel. Let $x, y$ and $z$ respectively be the number of lines in groups 1, 2, and 3. If no lines in group 3 go through the intersection of any two lines (in groups 1 and 2, of course), then the least number of lines required in order to have more than 2018 regions is ....

P8. It is known a frustum $ABCD.EFGH$ where $ABCD$ and $EFGH$ are squares with both planes being parallel. The length of the sides of $ABCD$ and $EFGH$ respectively are $6a$ and $3a$, and the height of the frustum is $3t$. Points $M$ and $N$ respectively are intersections of the diagonals of $ABCD$ and $EFGH$ and the line $MN$ is perpendicular to the plane $EFGH$. Construct the pyramids $M.EFGH$ and $N.ABCD$ and calculate the volume of the 3D figure which is the intersection of pyramids $N.ABCD$ and $M.EFGH$.

P9. Look at the arrangement of natural numbers in the following table. The position of the numbers is determined by their row and column numbers, and its diagonal (which, the sequence of numbers is read from the bottom left to the top right). As an example, the number $19$ is on the 3rd row, 4th column, and on the 6th diagonal. Meanwhile the position of the number $26$ is on the 3rd row, 5th column, and 7th diagonal.

(Image should be placed here, look at attachment.)

a) Determine the position of the number $2018$ based on its row, column, and diagonal.
b) Determine the average of the sequence of numbers whose position is on the "main diagonal" (quotation marks not there in the first place), which is the sequence of numbers read from the top left to the bottom right: 1, 5, 13, 25, ..., which the last term is the largest number that is less than or equal to $2018$.

P10. It is known that $A$ is the set of 3-digit integers not containing the digit $0$. Define a gadang number to be the element of $A$ whose digits are all distinct and the digits contained in such number are not prime, and (a gadang number leaves a remainder of 5 when divided by 7. If we pick an element of $A$ at random, what is the probability that the number we picked is a gadang number?
1 reply
somebodyyouusedtoknow
Nov 11, 2021
Rayholr123
2 hours ago
Indonesian Junior MO 2018 (Nationals), Day 1
somebodyyouusedtoknow   6
N 2 hours ago by Rayholr123
The problems are really difficult to find online, so here are the problems.

P1. It is known that two positive integers $m$ and $n$ satisfy $10n - 9m = 7$ dan $m \leq 2018$. The number $k = 20 - \frac{18m}{n}$ is a fraction in its simplest form.
a) Determine the smallest possible value of $k$.
b) If the denominator of the smallest value of $k$ is (equal to some number) $N$, determine all positive factors of $N$.
c) On taking one factor out of all the mentioned positive factors of $N$ above (specifically in problem b), determine the probability of taking a factor who is a multiple of 4.

I added this because my translation is a bit weird.
Indonesian Version

P2. Let the functions $f, g : \mathbb{R} \to \mathbb{R}$ be given in the following graphs.
Graph Construction Notes
Define the function $g \circ f$ with $(g \circ f)(x) = g(f(x))$ for all $x \in D_f$ where $D_f$ is the domain of $f$.
a) Draw the graph of the function $g \circ f$.
b) Determine all values of $x$ so that $-\frac{1}{2} \leq (g \circ f)(x) \leq 6$.

P3. The quadrilateral $ABCD$ has side lengths $AB = BC = 4\sqrt{3}$ cm and $CD = DA = 4$ cm. All four of its vertices lie on a circle. Calculate the area of quadrilateral $ABCD$.

P4. There exists positive integers $x$ and $y$, with $x < 100$ and $y > 9$. It is known that $y = \frac{p}{777} x$, where $p$ is a 3-digit number whose number in its tens place is 5. Determine the number/quantity of all possible values of $y$.

P5. The 8-digit number $\overline{abcdefgh}$ (the original problem does not have an overline, which I fixed) is arranged from the set $\{1, 2, 3, 4, 5, 6, 7, 8\}$. Such number satisfies $a + c + e + g \geq b + d + f + h$. Determine the quantity of different possible (such) numbers.

6 replies
somebodyyouusedtoknow
Nov 11, 2021
Rayholr123
2 hours ago
Minimize
lgx57   8
N 2 hours ago by lbh_qys
Minimize $\sqrt{\cos^2 x+(2-\sin x)^2}+\dfrac{1}{2}\sqrt{(\sqrt 3-\cos x)^2+(\sin x+1)^2}$
8 replies
lgx57
May 23, 2025
lbh_qys
2 hours ago
PHP trouble
SomeonecoolLovesMaths   0
3 hours ago
We have distributed two hundred balls into one hundred boxes with the restrictions that no box got more than one hundred balls, and each box got at least one. Prove that it is possible to find some boxes that together contain exactly one hundred balls.

Although I am aware of the proof, I do not understand the part where the only partial sum $ \equiv 0 \pmod {100}$ is $200$. Can someone give some kind of a commentary on the proof?
0 replies
SomeonecoolLovesMaths
3 hours ago
0 replies
Inequalities
lgx57   3
N 3 hours ago by lbh_qys
Let $a,b,c,d,e \ge 0$,$\sum \dfrac{1}{a+4}=1$.Prove that:
$$\sum \dfrac{a}{a^2+4} \le 1$$
Let $x,y,z>0$.Prove that:
$$\sum (y+z)\sqrt{\dfrac{yz}{(z+x)(y+x)}} \ge x+y+z$$
3 replies
lgx57
Yesterday at 3:55 PM
lbh_qys
3 hours ago
Algebraic Manipulation
Darealzolt   2
N 4 hours ago by lbh_qys
It is known that \(a,b \in \mathbb{R}\) that satisfies
\[
a^3+b^3=1957
\]\[
(a+b)(a+1)(b+1)=2014
\]Hence, find the value of \(a+b\)
2 replies
Darealzolt
Today at 4:01 AM
lbh_qys
4 hours ago
K,L,M and N lie on the same circle
StefanS   3
N Dec 27, 2013 by sayantanchakraborty
Source: Macedonia National Olympiad 2009 - Problem 2
Let $O$ be the centre of the incircle of $\triangle ABC$. Points $K,L$ are the intersection points of the circles circumscribed about triangles $BOC,AOC$ respectively with the bisectors of the angles at $A,B$ respectively $(K,L\not= O)$. Also $P$ is the midpoint of segment $KL$, $M$ is the reflection of $O$ with respect to $P$ and $N$ is the reflection of $O$ with respect to line $KL$. Prove that the points $K,L,M$ and $N$ lie on the same circle.
3 replies
StefanS
Apr 13, 2012
sayantanchakraborty
Dec 27, 2013
K,L,M and N lie on the same circle
G H J
Source: Macedonia National Olympiad 2009 - Problem 2
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StefanS
149 posts
#1 • 2 Y
Y by Adventure10, Mango247
Let $O$ be the centre of the incircle of $\triangle ABC$. Points $K,L$ are the intersection points of the circles circumscribed about triangles $BOC,AOC$ respectively with the bisectors of the angles at $A,B$ respectively $(K,L\not= O)$. Also $P$ is the midpoint of segment $KL$, $M$ is the reflection of $O$ with respect to $P$ and $N$ is the reflection of $O$ with respect to line $KL$. Prove that the points $K,L,M$ and $N$ lie on the same circle.
This post has been edited 1 time. Last edited by StefanS, Apr 13, 2012, 5:21 PM
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teps
131 posts
#2 • 2 Y
Y by StefanS, Adventure10
Simple angle chasing shows that $C$ is on the segment $KL$. Observe that $OKNL$ is a deltoid($NO\perp KL$), thus $\angle KNL=\angle KOL$. Also because $P$ is the midpoint of segment $KL$ and $OP=PM$, we have that $OKLM$ is a parallelogram (their diagonals divide in half), thus $\angle KML=\angle KOL$. $\blacksquare$
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jayme
9801 posts
#3 • 4 Y
Y by dr.mm, StefanS, AlastorMoody, Adventure10
Dear Mathlinkers,

an inverse lecture can be fruitful.
1. K, L are the A, B-excenters of ABC
2. O is the orthocenter of the excentral triangle of ABC
3. It is known that M and N are on the Bevan circle of ABC i.e. the circumcircle of the excentral triangle of ABC.

Sincerely
Jean-Louis
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sayantanchakraborty
505 posts
#4 • 1 Y
Y by Adventure10
Too easy for national olympiad.Anyways that was a good stuff by jayme.Good exploration.
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N Quick Reply
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