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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
Yesterday at 11:16 PM
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.

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[*]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
Yesterday at 11:16 PM
0 replies
k i Adding contests to the Contest Collections
dcouchman   1
N Apr 5, 2023 by v_Enhance
Want to help AoPS remain a valuable Olympiad resource? Help us add contests to AoPS's Contest Collections.

Find instructions and a list of contests to add here: https://artofproblemsolving.com/community/c40244h1064480_contests_to_add
1 reply
dcouchman
Sep 9, 2019
v_Enhance
Apr 5, 2023
k i Zero tolerance
ZetaX   49
N May 4, 2019 by NoDealsHere
Source: Use your common sense! (enough is enough)
Some users don't want to learn, some other simply ignore advises.
But please follow the following guideline:


To make it short: ALWAYS USE YOUR COMMON SENSE IF POSTING!
If you don't have common sense, don't post.


More specifically:

For new threads:


a) Good, meaningful title:
The title has to say what the problem is about in best way possible.
If that title occured already, it's definitely bad. And contest names aren't good either.
That's in fact a requirement for being able to search old problems.

Examples:
Bad titles:
- "Hard"/"Medium"/"Easy" (if you find it so cool how hard/easy it is, tell it in the post and use a title that tells us the problem)
- "Number Theory" (hey guy, guess why this forum's named that way¿ and is it the only such problem on earth¿)
- "Fibonacci" (there are millions of Fibonacci problems out there, all posted and named the same...)
- "Chinese TST 2003" (does this say anything about the problem¿)
Good titles:
- "On divisors of a³+2b³+4c³-6abc"
- "Number of solutions to x²+y²=6z²"
- "Fibonacci numbers are never squares"


b) Use search function:
Before posting a "new" problem spend at least two, better five, minutes to look if this problem was posted before. If it was, don't repost it. If you have anything important to say on topic, post it in one of the older threads.
If the thread is locked cause of this, use search function.

Update (by Amir Hossein). The best way to search for two keywords in AoPS is to input
[code]+"first keyword" +"second keyword"[/code]
so that any post containing both strings "first word" and "second form".


c) Good problem statement:
Some recent really bad post was:
[quote]$lim_{n\to 1}^{+\infty}\frac{1}{n}-lnn$[/quote]
It contains no question and no answer.
If you do this, too, you are on the best way to get your thread deleted. Write everything clearly, define where your variables come from (and define the "natural" numbers if used). Additionally read your post at least twice before submitting. After you sent it, read it again and use the Edit-Button if necessary to correct errors.


For answers to already existing threads:


d) Of any interest and with content:
Don't post things that are more trivial than completely obvious. For example, if the question is to solve $x^{3}+y^{3}=z^{3}$, do not answer with "$x=y=z=0$ is a solution" only. Either you post any kind of proof or at least something unexpected (like "$x=1337, y=481, z=42$ is the smallest solution). Someone that does not see that $x=y=z=0$ is a solution of the above without your post is completely wrong here, this is an IMO-level forum.
Similar, posting "I have solved this problem" but not posting anything else is not welcome; it even looks that you just want to show off what a genius you are.

e) Well written and checked answers:
Like c) for new threads, check your solutions at least twice for mistakes. And after sending, read it again and use the Edit-Button if necessary to correct errors.



To repeat it: ALWAYS USE YOUR COMMON SENSE IF POSTING!


Everything definitely out of range of common sense will be locked or deleted (exept for new users having less than about 42 posts, they are newbies and need/get some time to learn).

The above rules will be applied from next monday (5. march of 2007).
Feel free to discuss on this here.
49 replies
ZetaX
Feb 27, 2007
NoDealsHere
May 4, 2019
positive integers forming a perfect square
cielblue   0
17 minutes ago
Find all positive integers $n$ such that $2^n-n^2+1$ is a perfect square.
0 replies
cielblue
17 minutes ago
0 replies
Function equation
LeDuonggg   6
N 29 minutes ago by MathLuis
Find all functions $f: \mathbb{R^+} \rightarrow \mathbb{R^+}$ , such that for all $x,y>0$:
\[ f(x+f(y))=\dfrac{f(x)}{1+f(xy)}\]
6 replies
LeDuonggg
Yesterday at 2:59 PM
MathLuis
29 minutes ago
A nice and easy gem off of StackExchange
NamelyOrange   0
30 minutes ago
Source: https://math.stackexchange.com/questions/3818796/
Define $S$ as the set of all numbers of the form $2^i5^j$ for some nonnegative $i$ and $j$. Find (with proof) all pairs $(m,n)$ such that $m,n\in S$ and $m-n=1$.
0 replies
NamelyOrange
30 minutes ago
0 replies
at everystep a, b, c are replaced by a+\gcd(b,c), b+\gcd(a,c), c+\gcd(a,b)
NJAX   8
N an hour ago by Assassino9931
Source: 2nd Al-Khwarizmi International Junior Mathematical Olympiad 2024, Day2, Problem 8
Three positive integers are written on the board. In every minute, instead of the numbers $a, b, c$, Elbek writes $a+\gcd(b,c), b+\gcd(a,c), c+\gcd(a,b)$ . Prove that there will be two numbers on the board after some minutes, such that one is divisible by the other.
Note. $\gcd(x,y)$ - Greatest common divisor of numbers $x$ and $y$

Proposed by Sergey Berlov, Russia
8 replies
NJAX
May 31, 2024
Assassino9931
an hour ago
Increments and Decrements in Square Grid
ike.chen   23
N an hour ago by Andyexists
Source: ISL 2022/C3
In each square of a garden shaped like a $2022 \times 2022$ board, there is initially a tree of height $0$. A gardener and a lumberjack alternate turns playing the following game, with the gardener taking the first turn:
[list]
[*] The gardener chooses a square in the garden. Each tree on that square and all the surrounding squares (of which there are at most eight) then becomes one unit taller.
[*] The lumberjack then chooses four different squares on the board. Each tree of positive height on those squares then becomes one unit shorter.
[/list]
We say that a tree is majestic if its height is at least $10^6$. Determine the largest $K$ such that the gardener can ensure there are eventually $K$ majestic trees on the board, no matter how the lumberjack plays.
23 replies
ike.chen
Jul 9, 2023
Andyexists
an hour ago
4-var inequality
RainbowNeos   5
N 2 hours ago by RainbowNeos
Given $a,b,c,d>0$, show that
\[\frac{a}{b}+\frac{b}{c}+\frac{c}{d}+\frac{d}{a}\geq 4+\frac{8(a-c)^2}{(a+b+c+d)^2}.\]
5 replies
RainbowNeos
Yesterday at 9:31 AM
RainbowNeos
2 hours ago
Hard diophant equation
MuradSafarli   2
N 2 hours ago by MuradSafarli
Find all positive integers $x, y, z, t$ such that the equation

$$
2017^x + 6^y + 2^z = 2025^t
$$
is satisfied.
2 replies
MuradSafarli
3 hours ago
MuradSafarli
2 hours ago
An almost identity polynomial
nAalniaOMliO   6
N 2 hours ago by Primeniyazidayi
Source: Belarusian National Olympiad 2025
Let $n$ be a positive integer and $P(x)$ be a polynomial with integer coefficients such that $P(1)=1,P(2)=2,\ldots,P(n)=n$.
Prove that $P(0)$ is divisible by $2 \cdot 3 \cdot \ldots \cdot n$.
6 replies
nAalniaOMliO
Mar 28, 2025
Primeniyazidayi
2 hours ago
Euler's function
luutrongphuc   2
N 2 hours ago by KevinYang2.71
Find all real numbers \(\alpha\) such that for every positive real \(c\), there exists an integer \(n>1\) satisfying
\[
\frac{\varphi(n!)}{n^\alpha\,(n-1)!} \;>\; c.
\]
2 replies
luutrongphuc
5 hours ago
KevinYang2.71
2 hours ago
Wot n' Minimization
y-is-the-best-_   25
N 3 hours ago by john0512
Source: IMO SL 2019 A3
Let $n \geqslant 3$ be a positive integer and let $\left(a_{1}, a_{2}, \ldots, a_{n}\right)$ be a strictly increasing sequence of $n$ positive real numbers with sum equal to 2. Let $X$ be a subset of $\{1,2, \ldots, n\}$ such that the value of
\[
\left|1-\sum_{i \in X} a_{i}\right|
\]is minimised. Prove that there exists a strictly increasing sequence of $n$ positive real numbers $\left(b_{1}, b_{2}, \ldots, b_{n}\right)$ with sum equal to 2 such that
\[
\sum_{i \in X} b_{i}=1.
\]
25 replies
y-is-the-best-_
Sep 23, 2020
john0512
3 hours ago
Line AT passes through either S_1 or S_2
v_Enhance   88
N 3 hours ago by bjump
Source: USA December TST for 57th IMO 2016, Problem 2
Let $ABC$ be a scalene triangle with circumcircle $\Omega$, and suppose the incircle of $ABC$ touches $BC$ at $D$. The angle bisector of $\angle A$ meets $BC$ and $\Omega$ at $E$ and $F$. The circumcircle of $\triangle DEF$ intersects the $A$-excircle at $S_1$, $S_2$, and $\Omega$ at $T \neq F$. Prove that line $AT$ passes through either $S_1$ or $S_2$.

Proposed by Evan Chen
88 replies
v_Enhance
Dec 21, 2015
bjump
3 hours ago
Inequality with a,b,c
GeoMorocco   4
N 3 hours ago by Natrium
Source: Morocco Training
Let $   a,b,c   $ be positive real numbers such that : $   ab+bc+ca=3   $ . Prove that : $$\frac{\sqrt{1+a^2}}{1+ab}+\frac{\sqrt{1+b^2}}{1+bc}+\frac{\sqrt{1+c^2}}{1+ca}\ge \sqrt{\frac{3(a+b+c)}{2}}$$
4 replies
GeoMorocco
Apr 11, 2025
Natrium
3 hours ago
China Northern MO 2009 p4 CNMO
parkjungmin   1
N 3 hours ago by WallyWalrus
Source: China Northern MO 2009 p4 CNMO P4
The problem is too difficult.
1 reply
parkjungmin
Apr 30, 2025
WallyWalrus
3 hours ago
Polynomial Squares
zacchro   26
N 3 hours ago by Mathandski
Source: USA December TST for IMO 2017, Problem 3, by Alison Miller
Let $P, Q \in \mathbb{R}[x]$ be relatively prime nonconstant polynomials. Show that there can be at most three real numbers $\lambda$ such that $P + \lambda Q$ is the square of a polynomial.

Alison Miller
26 replies
zacchro
Dec 11, 2016
Mathandski
3 hours ago
Tiling rectangle with smaller rectangles.
MarkBcc168   61
N Apr 23, 2025 by YaoAOPS
Source: IMO Shortlist 2017 C1
A rectangle $\mathcal{R}$ with odd integer side lengths is divided into small rectangles with integer side lengths. Prove that there is at least one among the small rectangles whose distances from the four sides of $\mathcal{R}$ are either all odd or all even.

Proposed by Jeck Lim, Singapore
61 replies
MarkBcc168
Jul 10, 2018
YaoAOPS
Apr 23, 2025
Tiling rectangle with smaller rectangles.
G H J
G H BBookmark kLocked kLocked NReply
Source: IMO Shortlist 2017 C1
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ihatemath123
3446 posts
#55 • 1 Y
Y by bjump
WTF this is such an elegant problem

Color the rectangle black and white such that the corners are all black. Clearly, a small rectangle that satisfies the parity condition must have all black corners - equivalently, it has one more black square than white square. Other rectangles either contain an equal number of black squares and white squares, or more white squares than black. Since the large rectangle contains more black squares than white squares, we must have a small rectangle that satisfies the parity condition.
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noppi_kun
16 posts
#56
Y by
Nice problem.
This post has been edited 3 times. Last edited by noppi_kun, Jul 30, 2023, 10:46 PM
Reason: Sssssss
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ihatemath123
3446 posts
#57
Y by
noppi_kun wrote:
Each small rectangle must have an even area, and therefore $\mathcal{R}$ must also have an even area, which is a contradiction.

I believe this idea is correct, but there is no one else doing the same. Can someone please tell me if this is right or wrong

If a rectangle has odd area, we know for sure that the distance from the vertical sides of this rectangle to $\mathcal R$ are both even or odd; we also know for sure that the distance from the horizontal sides of this rectangle to $\mathcal R$ are both even or odd; however, we do not know that all four parities are the same.
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noppi_kun
16 posts
#58
Y by
ihatemath123 wrote:
noppi_kun wrote:
Each small rectangle must have an even area, and therefore $\mathcal{R}$ must also have an even area, which is a contradiction.

I believe this idea is correct, but there is no one else doing the same. Can someone please tell me if this is right or wrong

If a rectangle has odd area, we know for sure that the distance from the vertical sides of this rectangle to $\mathcal R$ are both even or odd; we also know for sure that the distance from the horizontal sides of this rectangle to $\mathcal R$ are both even or odd; however, we do not know that all four parities are the same.

I was wrong.. Thanks.
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LLL2019
834 posts
#59
Y by
Same as above. Here is my motivation for the coloring.

If the rectangle is $[0,a]\times [0,b]$ and we are considering $[x,x+c]\times [y,y+d]$, we wish to prove $c$, $d$ are odd and $x\equiv y\pmod 2$ is possible. Let us just focus on the first half. We can note that by weighting each cell by $1$, we see $\sum cd$ must be odd, so there must be some rectangle with $cd$ odd.

We can then think of doing a similar thing with $x\equiv y \pmod 2$. It is quite natural to think of labeling each $(x,y)$ with $x+y$. However, we see this does not work. Then, our next idea is to label each $(x,y)$ with $\delta(x+y\equiv 0\pmod 2)$, which works.
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bjump
1013 posts
#60
Y by
Fun combo :icecream:
Tile the grid like a checkerboard with corners shaded. We can observe that a small rectangle works iff its corners are all shaded, or in other words it has one more shaded squares than non shaded squares. Now since the sum of the areas of the rectangles inside $\mathcal{R}$ is odd. There must exist an odd by odd rectangle. If all odd rectangles had blank corners then, there would be more blank squares total than shaded squares, however the rectangle has more shaded squares a contradiction. So there must exist at least one among the small rectangles whose distances from the four sides of $\mathcal{R}$ are either all odd or all even.
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RedFireTruck
4221 posts
#61
Y by
Color the cells of the grid black and white in checkerboard pattern so that it has $4$ black corners. Iff the rectangle's distances from the four sides are all odd or all even, then it has $4$ black corners. Assume FTSOC that we do not use any rectangle with $4$ black corners in our dissection. Then we can only use rectangles with $2$ white and $2$ black corners, or $4$ white corners. In both cases, there are at least as many white squares as black squares, but in $\mathcal G$, we have more black squares than white squares. This is a contradiction, as desired.
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ezpotd
1262 posts
#62
Y by
Checkerboard tiling, with white at the corners. Clearly some rectangle must have white squares than black corners, if the rectangle has even area it is forced to be evenly split between white and black, so it is odd and all four corners are white. Taking distances from the white corners (every two adjacent directions have the same parity, so they all have the same parity) finishes.
This post has been edited 1 time. Last edited by ezpotd, Oct 2, 2024, 11:17 PM
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AshAuktober
1000 posts
#63
Y by
Solution with the following flavortext:
Quote:
A rectangular grid $\mathcal G$ has an odd number of square unit cells. The grid is dissected along its gridlines into several rectangles (with sides parallel to the grid). Prove that one of the rectangles $R$ in the dissection has the following property: the distance from $R$ to the four sides of $\mathcal G$ are either all odd or all even.
Color the unit squares of $\mathcal G$ in a chessboard fashion, with the bottom-left square being black.For a rectangle $ \mathcal R \in  \mathcal G$, define $B(\mathcal R)$ to be the number of black squares contained in it, and $W(\mathcal G)$ to be the number of white squares in it. Then the condition on our required rectangle $R$ is equivalent to $B(R) > W(R)$. So assume $B(R) \le W(R)$ for every rectangle. Then $$B(G) = \sum_{R \in P} B(R) \le \sum_{R \in P} W(R) = W(G),$$where $G$ refers to the rectangle covering the entire of $\mathcal G$, and $P$ refers to the partition. But this is impossible due to $G$ itself satisfying that $B(G) > W(G)$, contradiction. $\square$
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shendrew7
794 posts
#64
Y by
Checkerboard color the grid using black and white, where the corners are colored black. Notice a desired rectangle $R$ is equivalent to having odd dimensions and black corners, which must exist as there are more black squares than white squares in the grid. $\blacksquare$
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Maximilian113
574 posts
#65
Y by
Color the rectangle like a chessboard with alternating black and white colors, with the corners being black. If the desired result is not true, clearly each rectangle either has an equal amount of black and white squares, or one more white compared to black. Summing this up, it follows that the number of white squares is at least the number of black squares in the entire board, which is clearly a contradiction. QED
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Turtwig113
128 posts
#66
Y by
Split the large rectangle into unit squares and color them black and white in a checkerboard, with each corner being black. Then, any rectangle with more black squares than white squares would satisfy the condition because:

$\bullet$ Such a small rectangle would have odd area and thus odd side lengths, meaning the distances from each pair of parallel sides have the same parity (i.e. the distance from the top and bottom would both be odd or both be even)

$\bullet$ Such a small rectangle would also have the sum of the distances to the top and left sides of the large rectangle be even, since the top-left corner of the small rectangle would be black.

This means that the distances to the top and left sides of the large rectangle would either be both odd or both even, implying all the distances to the sides are all odd or all even. This clearly holds for some small rectangle, as the large rectangle has one more black square than white square, so there must be some small rectangle with more black squares than white squares, hence satisfying the condition.
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Bonime
36 posts
#67
Y by
$\textbf{C1 IMOSL 2017}$
Nice problem. I think I have an overcomplicated local solution:

The idea here is to apply an algorithm that transforms any general partition of the board into one consisting of only $2\times 2$, $2\times 1$, $1\times 2$ and $1\times 1$ rectangles. Suppose such a rectangle doesn't exists. Then, for any rectangle $a\times b$ which is contained in some division of $\mathcal{R}$, it follows the lemma

$\textbf{Lemma:}$ If a smaller rectangle $a\times b$ contained in $\mathcal{R}$ does not have the desired property, dividing it in a rectangle $a\times (b-2)$ and $a\times 2$ or in a $(a-2) \times b$ and $2 \times b$ will never generate rectangles with that.
$\textit{Proof:}$ By a $90^\circ$ rotation, it suffices to consider the first case. Note that the piece $a\times 2$ will never have, because $\mathcal{R}$ has odd sides, the distances of this piece from the right edge and the left edge of $\mathcal{R}$ have different parities. Now, note that since the original rectangle $a\times b$ don't have and the parity of the distance from the four $\mathcal{R}$ sides is preserved, the new piece $a\times (b-2)$ won't have it too $\blacksquare$.

Now, divide again all the rectangles of the board as much as possible using this process. Note that we'll get at that desired state. From here, it's easier to finish. Color $\mathcal{R}$ in a checkerboard pattern, such that all four corners are black. Note that the $2\times 2$, and the dominoes pieces cover an equal number of squares of each color. So, we need to have a $1\times 1$ covering a black square, but this implies all its distances to the sides of $\mathcal{R}$ have the same parity, then we're done. $\blacksquare$
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cursed_tangent1434
611 posts
#68
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Solution which is slightly sub-optimal than the official solution but the key idea is pretty much the same. It is pretty easy to see that all the rectangles must have sides along the lines dissecting $\mathcal{R}$ into a grid of $1 \times 1$ squares. Color the cells chessboard-style with the corners all white and number the white cells $1$ and $2$ with the top row $1$ and alternating numbers between rows. It is not hard to check that there are 6 types of rectangles (determined uniquely by their opposite corners) with the following parities of their areas.

$\bullet$ Type 1 - $1-B-B-1$ with even area
$\bullet$ Type 2 - $2-B-B-2$ with even area
$\bullet$ Type 3 - $1-B-2-B$ with even area
$\bullet$ Type 4 - $1-1-1-1$ with odd area
$\bullet$ Type 5 - $2-2-2-2$ with odd area
$\bullet$ Type 6 - $B-B-B-B$ with odd area

Now, assuming the contrary the dissection of $\mathcal{R}$ contains no rectangles of types 4 and 5 so it has an odd number of Type 6 rectangles. However, we now make the observation that for any $B$ square which is a corner of a sub-rectangle of $\mathcal{R}$, one of the cells neighboring it is a corner square of a sub-rectangle as white and this square is white. Thus, (counting with multiplicity in case of a row/column rectangle) the number of black corner cells in total is at most the number of white corner cells as black corner cells are never corners of $\mathcal{R}$. However, each rectangle of types 1,2 and 3 have at least as many black corners as white while Type 6 rectangles have more. This means our assumption implies strictly more black corner squares than white which is impossible so it is impossible to have solely Type 6 rectangles implying we must have atleast one rectangle of types 4 and 5 as desired.
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YaoAOPS
1535 posts
#69 • 1 Y
Y by Bonime
Take a coordinate grid. Then note that for a rectangle $\mathcal{D}$
\[
	f(\mathcal{D}) = \int\int_\mathcal{D} e^{\pi i(x+y)}
\]is nonzero iff it has all odd side lengths. Since $f(\mathcal{R})$ is nonzero, the $f$ of one of the rectangles in the partition must also be nonzero.
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