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k a April Highlights and 2025 AoPS Online Class Information
jlacosta   0
Apr 2, 2025
Spring is in full swing and summer is right around the corner, what are your plans? At AoPS Online our schedule has new classes starting now through July, so be sure to keep your skills sharp and be prepared for the Fall school year! Check out the schedule of upcoming classes below.

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[*]April 22nd (Webinar), 4:00pm PT/7:00pm ET, Competitive Programming at AoPS (USACO).[/list]
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0 replies
jlacosta
Apr 2, 2025
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
1 viewing
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
Inequalities
sqing   14
N 14 minutes ago by sqing
Let $   a,b    $ be reals such that $  a^2+ab+b^2 =3$ . Prove that
$$ \frac{4}{ 3}\geq \frac{1}{ a^2+5 }+ \frac{1}{ b^2+5 }+ab \geq -\frac{11}{4 }$$$$ \frac{13}{ 4}\geq \frac{1}{ a^2+5 }+ \frac{1}{ b^2+5 }+ab \geq -\frac{2}{3 }$$$$ \frac{3}{ 2}\geq  \frac{1}{ a^4+3 }+ \frac{1}{ b^4+3 }+ab \geq -\frac{17}{6 }$$$$ \frac{19}{ 6}\geq  \frac{1}{ a^4+3 }+ \frac{1}{ b^4+3 }-ab \geq -\frac{1}{2}$$Let $   a,b    $ be reals such that $  a^2-ab+b^2 =1 $ . Prove that
$$ \frac{3}{ 2}\geq \frac{1}{ a^2+3 }+ \frac{1}{ b^2+3 }+ab \geq \frac{4}{15 }$$$$ \frac{14}{ 15}\geq \frac{1}{ a^2+3 }+ \frac{1}{ b^2+3 }-ab \geq -\frac{1}{2 }$$$$ \frac{3}{ 2}\geq \frac{1}{ a^4+3 }+ \frac{1}{ b^4+3 }+ab \geq \frac{13}{42 }$$$$ \frac{41}{ 42}\geq \frac{1}{ a^4+3 }+ \frac{1}{ b^4+3 }-ab \geq -\frac{1}{2 }$$
14 replies
sqing
Apr 16, 2025
sqing
14 minutes ago
Geometry
German_bread   1
N 24 minutes ago by vanstraelen
A semicircle k with radius r is constructed over the line segment ST. Let D be a point on the line segment ST that is different from S and T. The two squares ABCD and DEF G lie in the half-plane of the semicircle such that points B and F lie on the semicircle k and points S, C, D, E, and T lie on a straight line in that order. (Points A and/or G can also lie outside the semicircle if necessary.)
Investigate whether the sum of the areas of the squares ABCD and DEFG depends on the position of point D on the line segment ST.

German math olympiad, class 9, 2022
1 reply
German_bread
2 hours ago
vanstraelen
24 minutes ago
Reflected point lies on radical axis
Mahdi_Mashayekhi   2
N an hour ago by gghx
Source: Iran 2025 second round P4
Given is an acute and scalene triangle $ABC$ with circumcenter $O$. $BO$ and $CO$ intersect the altitude from $A$ to $BC$ at points $P$ and $Q$ respectively. $X$ is the circumcenter of triangle $OPQ$ and $O'$ is the reflection of $O$ over $BC$. $Y$ is the second intersection of circumcircles of triangles $BXP$ and $CXQ$. Show that $X,Y,O'$ are collinear.
2 replies
Mahdi_Mashayekhi
an hour ago
gghx
an hour ago
hard problem (to me)
kjhgyuio   1
N an hour ago by Lankou
........
1 reply
kjhgyuio
Today at 5:04 AM
Lankou
an hour ago
High deg ine
m4thbl3nd3r   0
an hour ago
Let $a,b,c \ge 0$ s.t $a+b+c=2$. Prove that $$(a^3+b^3)(b^3+c^3)(c^3+a^3)\le 2$$
0 replies
1 viewing
m4thbl3nd3r
an hour ago
0 replies
Similar triangles formed by angular condition
Mahdi_Mashayekhi   1
N an hour ago by gghx
Source: Iran 2025 second round P3
Point $P$ lies inside of scalene triangle $ABC$ with incenter $I$ such that $:$
$$ 2\angle ABP = \angle BCA , 2\angle ACP = \angle CBA $$Lines $PB$ and $PC$ intersect line $AI$ respectively at $B'$ and $C'$. Line through $B'$ parallel to $AB$ intersects $BI$ at $X$ and line through $C'$ parallel to $AC$ intersects $CI$ at $Y$. Prove that triangles $PXY$ and $ABC$ are similar.
1 reply
Mahdi_Mashayekhi
2 hours ago
gghx
an hour ago
RMO KV 2024 Q5
SomeonecoolLovesMaths   4
N an hour ago by g0USinsane777
Source: RMO KV 2024 Q5
Let $ABC$ be a triangle with $\angle ABC  = 20^{\circ}$ and $\angle ACB  = 40^{\circ}$. Let $D$ be a point on $BC$ such that $\angle BAD  = \angle DAC$. Let the incircle of triangle $ABC$ touch $BC$ at $E$. Prove that $BD = 2 \cdot CE$.
4 replies
SomeonecoolLovesMaths
Nov 3, 2024
g0USinsane777
an hour ago
Indonesia Regional MO 2019 Part A
parmenides51   22
N 2 hours ago by SomeonecoolLovesMaths
Indonesia Regional MO
Year 2019 Part A

Time: 90 minutes Rules


p1. In the bag there are $7$ red balls and $8$ white balls. Audi took two balls at once from inside the bag. The chance of taking two balls of the same color is ...


p2. Given a regular hexagon with a side length of $1$ unit. The area of the hexagon is ...


p3. It is known that $r, s$ and $1$ are the roots of the cubic equation $x^3 - 2x + c = 0$. The value of $(r-s)^2$ is ...


p4. The number of pairs of natural numbers $(m, n)$ so that $GCD(n,m) = 2$ and $LCM(m,n) = 1000$ is ...


p5. A data with four real numbers $2n-4$, $2n-6$, $n^2-8$, $3n^2-6$ has an average of $0$ and a median of $9/2$. The largest number of such data is ...


p6. Suppose $a, b, c, d$ are integers greater than $2019$ which are four consecutive quarters of an arithmetic row with $a <b <c <d$. If $a$ and $d$ are squares of two consecutive natural numbers, then the smallest value of $c-b$ is ...


p7. Given a triangle $ABC$, with $AB = 6$, $AC = 8$ and $BC = 10$. The points $D$ and $E$ lies on the line segment $BC$. with $BD = 2$ and $CE = 4$. The measure of the angle $\angle DAE$ is ...


p8. Sequqnce of real numbers $a_1,a_2,a_3,...$ meet $\frac{na_1+(n-1)a_2+...+2a_{n-1}+a_n}{n^2}=1$ for each natural number $n$. The value of $a_1a_2a_3...a_{2019}$ is ....


p9. The number of ways to select four numbers from $\{1,2,3, ..., 15\}$ provided that the difference of any two numbers at least $3$ is ...


p10. Pairs of natural numbers $(m , n)$ which satisfies $$m^2n+mn^2 +m^2+2mn = 2018m + 2019n + 2019$$are as many as ...


p11. Given a triangle $ABC$ with $\angle ABC =135^o$ and $BC> AB$. Point $D$ lies on the side $BC$ so that $AB=CD$. Suppose $F$ is a point on the side extension $AB$ so that $DF$ is perpendicular to $AB$. The point $E$ lies on the ray $DF$ such that $DE> DF$ and $\angle ACE = 45^o$. The large angle $\angle AEC$ is ...


p12. The set of $S$ consists of $n$ integers with the following properties: For every three different members of $S$ there are two of them whose sum is a member of $S$. The largest value of $n$ is ....


p13. The minimum value of $\frac{a^2+2b^2+\sqrt2}{\sqrt{ab}}$ with $a, b$ positive reals is ....


p14. The polynomial P satisfies the equation $P (x^2) = x^{2019} (x+ 1) P (x)$ with $P (1/2)= -1$ is ....


p15. Look at a chessboard measuring $19 \times 19$ square units. Two plots are said to be neighbors if they both have one side in common. Initially, there are a total of $k$ coins on the chessboard where each coin is only loaded exactly on one square and each square can contain coins or blanks. At each turn. You must select exactly one plot that holds the minimum number of coins in the number of neighbors of the plot and then you must give exactly one coin to each neighbor of the selected plot. The game ends if you are no longer able to select squares with the intended conditions. The smallest number of $k$ so that the game never ends for any initial square selection is ....
22 replies
parmenides51
Nov 11, 2021
SomeonecoolLovesMaths
2 hours ago
Same radius geo
ThatApollo777   1
N 2 hours ago by CHESSR1DER
Source: Own
Classify all possible quadrupes of $4$ distinct points in a plane such the circumradius of any $3$ of them is the same.
1 reply
ThatApollo777
5 hours ago
CHESSR1DER
2 hours ago
one cyclic formed by two cyclic
CrazyInMath   37
N 2 hours ago by G81928128
Source: EGMO 2025/3
Let $ABC$ be an acute triangle. Points $B, D, E$, and $C$ lie on a line in this order and satisfy $BD = DE = EC$. Let $M$ and $N$ be the midpoints of $AD$ and $AE$, respectively. Suppose triangle $ADE$ is acute, and let $H$ be its orthocentre. Points $P$ and $Q$ lie on lines $BM$ and $CN$, respectively, such that $D, H, M,$ and $P$ are concyclic and pairwise different, and $E, H, N,$ and $Q$ are concyclic and pairwise different. Prove that $P, Q, N,$ and $M$ are concyclic.
37 replies
CrazyInMath
Apr 13, 2025
G81928128
2 hours ago
Iran second round 2025-q1
mohsen   0
2 hours ago
Find all positive integers n>2 such that sum of n and any of its prime divisors is a perfect square.
0 replies
mohsen
2 hours ago
0 replies
Maximizing the Sum of Minimum Differences in Permutations
chinawgp   0
2 hours ago
Problem Statement

Given a positive integer n \geq 3 , consider a permutation \pi = (a_1, a_2, \dots, a_n) of \{1, 2, \dots, n\} . For each i ( 1 \leq i \leq n-1 ), define d_i as the minimum absolute difference between a_i and any subsequent element a_j ( j > i ), i.e.,
d_i = \min \{ |a_i - a_j| \mid j > i \}.

Let S_n denote the maximum possible sum of d_i over all permutations of \{1, \dots, n\} , i.e.,
S_n = \max_{\pi} \sum_{i=1}^{n-1} d_i.

Proposed Construction

I found a method to construct a permutation that seems to maximize \sum d_i :
1. Fix a_{n-1} = 1 and a_n = n .
2. For each i (from n-2 down to 1 ):
- Sort a_{i+1}, a_{i+2}, \dots, a_n in increasing order.
- Compute the gaps between consecutive elements.
- Place a_i in the middle of the largest gap (if the gap has even length, choose the smaller midpoint).

Partial Results

1. I can prove that 1 and n must occupy the last two positions. Otherwise, moving either 1 or n further right does not decrease \sum d_i .
2. The construction greedily maximizes each d_i locally, but I’m unsure if this ensures global optimality.

Request for Help

- Does this construction always yield the maximum S_n ?
- If yes, how can we rigorously prove it? (Induction? Exchange arguments?)
- If no, what is the correct approach?

Observations:
- The construction works for small n (e.g., n=3,4,5,...,12 ).
- The problem resembles optimizing "minimum gaps" in permutations.

Any insights or references would be greatly appreciated!
0 replies
chinawgp
2 hours ago
0 replies
FE inequality from Iran
mojyla222   1
N 2 hours ago by bin_sherlo
Source: Iran 2025 second round P5
Find all functions $f:\mathbb{R}^+ \to \mathbb{R}$ such that for all $x,y,z>0$
$$
3(x^3+y^3+z^3)\geq f(x+y+z)\cdot f(xy+yz+xz) \geq (x+y+z)(xy+yz+xz).
$$
1 reply
+1 w
mojyla222
3 hours ago
bin_sherlo
2 hours ago
True or false?
Nguyenngoctu   3
N 3 hours ago by MathsII-enjoy
Let $a,b,c > 0$ such that $ab + bc + ca = 3$. Prove that ${a^3} + {b^3} + {c^3} \ge {a^3}{b^3} + {b^3}{c^3} + {c^3}{a^3}$
3 replies
Nguyenngoctu
Nov 17, 2017
MathsII-enjoy
3 hours ago
How to prove one-one function
Vulch   7
N Yesterday at 1:23 PM by SomeonecoolLovesMaths
Hello everyone,
I am learning functional equations.
To prove the below problem one -one function,I have taken two non-negative real numbers $ (1,2)$ from the domain $\Bbb R_{*},$ and put those numbers into the given function f(x)=1/x.It gives us 1=1/2.But it's not true.So ,it can't be one-one function.But in the answer,it is one-one function.Would anyone enlighten me where is my fault? Thank you!
7 replies
Vulch
Apr 11, 2025
SomeonecoolLovesMaths
Yesterday at 1:23 PM
How to prove one-one function
G H J
G H BBookmark kLocked kLocked NReply
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Vulch
2682 posts
#1
Y by
Hello everyone,
I am learning functional equations.
To prove the below problem one -one function,I have taken two non-negative real numbers $ (1,2)$ from the domain $\Bbb R_{*},$ and put those numbers into the given function f(x)=1/x.It gives us 1=1/2.But it's not true.So ,it can't be one-one function.But in the answer,it is one-one function.Would anyone enlighten me where is my fault? Thank you!
Attachments:
This post has been edited 3 times. Last edited by Vulch, Apr 11, 2025, 8:07 PM
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douqile
26 posts
#2
Y by
allright
Z K Y
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anticodon
138 posts
#3
Y by
(1,2) is not a point on the function
Z K Y
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SomeonecoolLovesMaths
3192 posts
#4
Y by
Do you know what one-one and onto functions are?

By definition one-one functions, also known as injective functions, are functions such that for any $2$ distinct inputs, they cannot give the same output.

For example,
Say $f: A \longrightarrow B$ is a function. $a,b \in A$. If $f$ is one-one then for any choice of $a$ and $b$ such that $a \neq b$, $f(a)$ cannot be equal to $f(b)$, that is, $f(a) \neq f(b)$.

So for example in your example, $f(1) = \frac{1}{1}$ and $f(2) = \frac{1}{2}$. As $f(1) \neq f(2)$, $(a,b) = (1,2)$ is not a correct example to contradict injectivity of the function.

In this prove, we will FTSOC assume that the function is not one-one. So there must exist a pair $(a,b) \in {\mathbb{R}^{+}}^2$ such that $a \neq b$ and $f(a) = f(b)$. But that means that $\frac{1}{a} = \frac{1}{b}$. And since both $a,b$ are not $0$, $a=b$. This contradicts our assumption and hence $f$ must be injective.

Now can you show its surjectivity?
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Vulch
2682 posts
#5
Y by
Would you show its subjectivity?
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jasperE3
11222 posts
#6
Y by
Vulch wrote:
Would you show its subjectivity?

Let $x\in\mathbb R*$ be any nonzero real number, for surjectivity we need to show that there is a $y\in\mathbb R*$ with $f(y)=x$. We can choose $y=\frac1x$, since clearly $f\left(\frac1x\right)=x$ and $\frac1x\in\mathbb R*$.
Z K Y
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Vulch
2682 posts
#7
Y by
Solve the following problem:
Attachments:
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SomeonecoolLovesMaths
3192 posts
#8
Y by
Vulch wrote:
Solve the following problem:

What have you done so far?

Of course it is not one-one as $f(0.5) = f(1) = 1$.
Of course it is not onto as there is no $k$ such that $f(k) = 0.5$.
Z K Y
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