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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
31 minutes ago
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)!

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[*]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
31 minutes ago
0 replies
Do not try to bash on beautiful geometry
ItzsleepyXD   6
N 10 minutes ago by Assassino9931
Source: Own , Mock Thailand Mathematic Olympiad P9
Let $ABC$be triangle with point $D,E$ and $F$ on $BC,AB,CA$
such that $BE=CF$ and $E,F$ are on the same side of $BC$
Let $M$ be midpoint of segment $BC$ and $N$ be midpoint of segment $EF$
Let $G$ be intersection of $BF$ with $CE$ and $\dfrac{BD}{DC}=\dfrac{AC}{AB}$
Prove that $MN\parallel DG$
6 replies
ItzsleepyXD
Yesterday at 9:30 AM
Assassino9931
10 minutes ago
PAMO 2017 Shortlst: Sum of maxima of adjacent pairs in permutation
DylanN   1
N 11 minutes ago by MelonGirl
Source: 2017 Pan-African Shortlist - I4
Find the maximum and minimum of the expression
\[
    \max(a_1, a_2) + \max(a_2, a_3), + \dots + \max(a_{n-1}, a_n) + \max(a_n, a_1),
\]where $(a_1, a_2, \dots, a_n)$ runs over the set of permutations of $(1, 2, \dots, n)$.
1 reply
DylanN
May 5, 2019
MelonGirl
11 minutes ago
Bijective quartic modulo p
DottedCaculator   12
N an hour ago by MathLuis
Source: ELMO 2024/6
For a prime $p$, let $\mathbb{F}_p$ denote the integers modulo $p$, and let $\mathbb{F}_p[x]$ be the set of polynomials with coefficients in $\mathbb{F}_p$. Find all $p$ for which there exists a quartic polynomial $P(x) \in \mathbb{F}_p[x]$ such that for all integers $k$, there exists some integer $\ell$ such that $P(\ell) \equiv k \pmod p$. (Note that there are $p^4(p-1)$ quartic polynomials in $\mathbb{F}_p[x]$ in total.)

Aprameya Tripathy
12 replies
+1 w
DottedCaculator
Jun 21, 2024
MathLuis
an hour ago
No More than √㏑x㏑㏑x Digits
EthanWYX2009   3
N an hour ago by MathisWow
Source: 2024 April 谜之竞赛-3
Let $f(x)\in\mathbb Z[x]$ have positive integer leading coefficient. Show that there exists infinte positive integer $x,$ such that the number of digit that doesn'r equal to $9$ is no more than $\mathcal O(\sqrt{\ln x\ln\ln x}).$

Created by Chunji Wang, Zhenyu Dong
3 replies
EthanWYX2009
Mar 24, 2025
MathisWow
an hour ago
If it is an integer then perfect square
Ecrin_eren   0
3 hours ago


"Let a, b, c, d be non-zero digits, and let abcd and dcba represent four-digit numbers.

Show that if the number abcd / dcba is an integer, then that integer is a perfect square."



0 replies
Ecrin_eren
3 hours ago
0 replies
Sum of arctan
Ecrin_eren   1
N 3 hours ago by Shan3t


Find the value of the sum:
sum from n = 0 to infinity of arctan(k / (n² + kn + 1))


1 reply
Ecrin_eren
3 hours ago
Shan3t
3 hours ago
Inequality
Ecrin_eren   0
3 hours ago


Let a, b, c be positive real numbers. Prove the inequality:

sqrt(a² - ab + b²) + sqrt(b² - bc + c²) ≥ sqrt(a² + ac + c²)



0 replies
Ecrin_eren
3 hours ago
0 replies
Cool vieta sum
Kempu33334   6
N 5 hours ago by Lankou
Let the roots of \[\mathcal{P}(x) = x^{108}+x^{102}+x^{96}+2x^{54}+3x^{36}+4x^{24}+5x^{18}+6\]be $r_1, r_2, \dots, r_{108}$. Find \[\dfrac{r_1^6+r_2^6+\dots+r_{108}^6}{r_1^6r_2^6+r_1^6r_3^6+\dots+r_{107}^6r_{108}^6}\]without Newton Sums.
6 replies
Kempu33334
Yesterday at 11:44 PM
Lankou
5 hours ago
đề hsg toán
akquysimpgenyabikho   3
N 6 hours ago by Lankou
làm ơn giúp tôi giải đề hsg

3 replies
akquysimpgenyabikho
Apr 27, 2025
Lankou
6 hours ago
A problem with a rectangle
Raul_S_Baz   13
N Today at 4:38 PM by undefined-NaN
On the sides AB and AD of the rectangle ABCD, points M and N are taken such that MB = ND. Let P be the intersection of BN and CD, and Q be the intersection of DM and CB. How can we prove that PQ || MN?
IMAGE
13 replies
Raul_S_Baz
Apr 26, 2025
undefined-NaN
Today at 4:38 PM
Find the domain and range of $f(x)=2-|x-5|.$
Vulch   1
N Today at 12:13 PM by Mathzeus1024
Find the domain and range of $f(x)=2-|x-5|.$
1 reply
Vulch
Today at 2:07 AM
Mathzeus1024
Today at 12:13 PM
nice problem
teomihai   1
N Today at 11:58 AM by Royal_mhyasd
Let set $A =\{0,1,2,3,...,n\}$ , where $n$ it is positiv ,integer number.
How many subsets of A contain at least one odd number?
1 reply
teomihai
Today at 11:46 AM
Royal_mhyasd
Today at 11:58 AM
(14n+25)/(2n+1) 'is a perfect square - Portugal OPM 2017 p1
parmenides51   4
N Today at 10:03 AM by Namisgood
Determine all integer values of n for which the number $\frac{14n+25}{2n+1}$ 'is a perfect square.
4 replies
parmenides51
May 15, 2024
Namisgood
Today at 10:03 AM
Inequalities
sqing   4
N Today at 9:46 AM by sqing
Let $ a,b,c>0 . $ Prove that
$$ \left(1 +\frac{a}{b}\right)\left(1+\frac{b}{c}\right)\left(1+\frac{c}{a}\right )\geq 4\left(\frac{a+b}{b+c}+ \frac{b+c}{a+b}\right)$$$$ \left(1 +\frac{a}{b}\right)\left(1+\frac{b}{c}\right)\left(1+\frac{c}{a}\right )\geq \frac{32}{9}\left(\frac{a+b}{b+c}+ \frac{c+a}{a+b}\right)$$$$ \left(1 +\frac{a}{b}\right)\left(1+\frac{b}{c}\right)\left(1+\frac{c}{a}\right )\geq  \frac{8}{3}\left(  \frac{a+b}{b+c}+ \frac{b+c}{c+a}+ \frac{c+a}{a+b}\right)$$$$ \left(1 +\frac{a^2}{b^2}\right)\left(1+\frac{b^2}{c^2}\right)\left(1+\frac{c^2}{a^2}\right )\geq \frac{8}{3}\left(  \frac{a^2+bc}{b^2+ca}+\frac{b^2+ca}{c^2+ab}+\frac{c^2+ab}{a^2+bc}\right)$$
4 replies
sqing
Today at 12:20 AM
sqing
Today at 9:46 AM
Paint and Optimize: A Grid Strategy Problem
mojyla222   2
N Apr 21, 2025 by sami1618
Source: Iran 2025 second round p2
Ali and Shayan are playing a turn-based game on an infinite grid. Initially, all cells are white. Ali starts the game, and in the first turn, he colors one unit square black. In the following turns, each player must color a white square that shares at least one side with a black square. The game continues for exactly 2808 turns, after which each player has made 1404 moves. Let $A$ be the set of black cells at the end of the game. Ali and Shayan respectively aim to minimize and maximise the perimeter of the shape $A$ by playing optimally. (The perimeter of shape $A$ is defined as the total length of the boundary segments between a black and a white cell.)

What are the possible values of the perimeter of $A$, assuming both players play optimally?
2 replies
mojyla222
Apr 20, 2025
sami1618
Apr 21, 2025
Paint and Optimize: A Grid Strategy Problem
G H J
G H BBookmark kLocked kLocked NReply
Source: Iran 2025 second round p2
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mojyla222
103 posts
#1
Y by
Ali and Shayan are playing a turn-based game on an infinite grid. Initially, all cells are white. Ali starts the game, and in the first turn, he colors one unit square black. In the following turns, each player must color a white square that shares at least one side with a black square. The game continues for exactly 2808 turns, after which each player has made 1404 moves. Let $A$ be the set of black cells at the end of the game. Ali and Shayan respectively aim to minimize and maximise the perimeter of the shape $A$ by playing optimally. (The perimeter of shape $A$ is defined as the total length of the boundary segments between a black and a white cell.)

What are the possible values of the perimeter of $A$, assuming both players play optimally?
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YaoAOPS
1533 posts
#2
Y by
The answer is $2 \cdot 1404 + 6 = 2814$.

Ali's strategy is to place their second cell to form an $L$, and then always fill black cells with two neighbors. This only breaks down if the currently colored black squares are an $2a \times b$ rectangle. Then if we consider the last time occurs then there's at most $2a + 2b + 2\cdot (1404 - ab) < 2814$. Else, Ali adds at most $6$ to the perimeter and Shayan $2$ each turn giving the result.

Shayans strategy is to place each of their moves to add $2$, with Ali's first two moves must add $2$ and $4$ the result follows.
Z K Y
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sami1618
901 posts
#3
Y by
Answer: $2814$

Solution. We show two strategies, one for Ali to ensure that the perimeter of $A$ is at most $2814$, regardless of how Shayan plays, and one for Shayan to ensure that the perimeter of $A$ is at least $2814$, regardless of how Ali plays. This is clearly sufficient to show that if both players play optimally, the perimeter of $A$ will be $2814$.

Strategy for Ali: Let $I$ be an index starting at $0$. Each time someone colors a square increase $I$ be the number of black squares that are adjacent to that square. For Ali's second move, he should play so that the black squares form an $L$-tromino. Thus after three turns, $I=2$. We claim that for each of the following $1402$ pairs of turns where Shayan plays and then Ali plays, Ali can guarantee that $I$ increases by at least $3$ during their pair of turns. Each move will increase $I$ by at least $1$. If Shayan's move increases $I$ by at least $2$, then we are done. If Shayan's move increases $I$ by exactly $1$, then the resulting colored squares can not form a rectangle (notice the rectangle can not be a stick because of Ali's second move). Then it is always possible for Ali to color a square that will increase $I$ by at least $2$ (otherwise the shape cannot have holes and the boundary would be rectangular). Thus after these $1402$ pairs of turns, $I$ has increased by at least $4206$. For Shayan's final move, $I$ will increase by at least one more. To finish, $$\text{Perimeter}(A)=4\cdot \#\text{black squares}-2\cdot I\leq 4\cdot 2808-2\cdot 4209=2814$$
Strategy for Shayan: Let $I$ be an index counting the sum of the height and width of the smallest axis-aligned rectangle containing all of the black cells. After Ali's second move, it will always be that $I=4$. Then for each of Shayan's next moves he can always increase $I$ by $1$ by coloring a square directly below one of the lowest existing black squares. Thus by the end, Shayan can ensure that $I$ is at least $1407$. Then shooting a laser through each unit on the perimeter of the rectangle must hit different edges along the perimeter of $A$. Thus to finish, $$\text{Perimeter}(A)\geq 2I\geq 2814$$
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