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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
Thursday 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.

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 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
Thursday 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
Some free permutation
IndoMathXdZ   23
N 32 minutes ago by Jupiterballs
Source: ISL 2020 N7
Let $\mathcal{S}$ be a set consisting of $n \ge 3$ positive integers, none of which is a sum of two other distinct members of $\mathcal{S}$. Prove that the elements of $\mathcal{S}$ may be ordered as $a_1, a_2, \dots, a_n$ so that $a_i$ does not divide $a_{i - 1} + a_{i + 1}$ for all $i = 2, 3, \dots, n - 1$.
23 replies
IndoMathXdZ
Jul 20, 2021
Jupiterballs
32 minutes ago
Inspired by old results
sqing   5
N an hour ago by Novmath
Source: Own
Let $ a,b>0 , a^2+b^2+ab+a+b=5 . $ Prove that
$$ \frac{ 1 }{a+b+ab+1}+\frac{6}{a^2+b^2+ab+1}\geq \frac{7}{4}$$$$ \frac{ 1 }{a+b+ab+1}+\frac{1}{a^2+b^2+ab+1}\geq \frac{1}{2}$$$$  \frac{41}{a+b+2}+\frac{ab}{a^3+b^3+2} \geq \frac{21}{2}$$
5 replies
sqing
Apr 29, 2025
Novmath
an hour ago
Tangency geo
Assassino9931   3
N an hour ago by Assassino9931
Source: RMM Shortlist 2024 G1
Let $ABC$ be an acute triangle with $\angle ABC > 45^{\circ}$ and $\angle ACB > 45^{\circ}$. Let $M$ be the midpoint of the side $BC$. The circumcircle of triangle $ABM$ intersects the side $AC$ again at $X\neq A$ and the circumcircle of triangle $ACM$ intersects the side $AB$ again at $Y\neq A$. The point $P$ lies on the perpendicular bisector of the segment $BC$ so that the points $P$ and $A$ lie on the same side of $XY$ and $\angle XPY = 90^{\circ} + \angle BAC$. Prove that the circumcircles of triangles $BPY$ and $CPX$ are tangent.
3 replies
1 viewing
Assassino9931
Yesterday at 10:57 PM
Assassino9931
an hour ago
Problem 6
SlovEcience   0
2 hours ago
Given two points A and B on the unit circle. The tangents to the circle at A and B intersect at point P. Then:
\[ p = \frac{2ab}{a + b} \], \[ p, a, b \in \mathbb{C} \]
0 replies
SlovEcience
2 hours ago
0 replies
cubefree divisibility
DottedCaculator   59
N 2 hours ago by SimplisticFormulas
Source: 2021 ISL N1
Find all positive integers $n\geq1$ such that there exists a pair $(a,b)$ of positive integers, such that $a^2+b+3$ is not divisible by the cube of any prime, and $$n=\frac{ab+3b+8}{a^2+b+3}.$$
59 replies
DottedCaculator
Jul 12, 2022
SimplisticFormulas
2 hours ago
Summation
Saucepan_man02   0
2 hours ago
If $P = \sum_{r=1}^{50} \sum_{k=1}^{r} (-1)^{r-1} \frac{\binom{50}{r}}{k}$, then find the value of $P$.

Ans
0 replies
Saucepan_man02
2 hours ago
0 replies
Interesting inequalities
sqing   2
N 2 hours ago by sqing
Source: Own
Let $ a,b,c\geq 0 ,2a +ab +abc \geq 9. $ Prove that
$$a+b+c  \geq 4$$$$a+b+\frac{1}{4}c  \geq \frac{13}{4}$$Let $ a,b,c\geq 0 ,2a +ab +4abc \geq 9. $ Prove that
$$a+b+c+abc  \geq 4$$$$a+b+4c   \geq 4$$$$a+b+c  \geq \frac{13}{4}$$$$a+\frac{3}{2}b+4c   \geq 3(\sqrt{6}-1)$$$$a+\frac{9}{4}b+4c \geq \frac{9}{2}$$$$a+\frac{4}{3}b+4c   \geq 4\sqrt 3-\frac{8}{3}$$
2 replies
sqing
3 hours ago
sqing
2 hours ago
Equation of Matrices which have same rank
PureRun89   3
N 3 hours ago by pi_quadrat_sechstel
Source: Gazeta Mathematica

Let $A,B \in \mathbb{C}_{n \times n}$ and $rank(A)=rank(B)$.
Given that there exists positive integer $k$ such that
$$A^{k+1} B^k=A.$$Prove that
$$B^{k+1} A^k=B.$$
(Note: The submition of the problem is end so I post this)
3 replies
PureRun89
May 18, 2023
pi_quadrat_sechstel
3 hours ago
A convex pentagon has rational sides and equal angles
Valentin Vornicu   2
N 3 hours ago by Qing-Cloud
Source: Balkan MO 2001, problem 2
A convex pentagon $ABCDE$ has rational sides and equal angles. Show that it is regular.
2 replies
Valentin Vornicu
Apr 24, 2006
Qing-Cloud
3 hours ago
Hard diophant equation
MuradSafarli   3
N 3 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.
3 replies
MuradSafarli
Yesterday at 6:12 PM
MuradSafarli
3 hours ago
D1023 : MVT 2.0
Dattier   1
N 4 hours ago by Dattier
Source: les dattes à Dattier
Let $f \in C(\mathbb R)$ derivable on $\mathbb R$ with $$\forall x \in \mathbb R,\forall h \geq 0, f(x)-3f(x+h)+3f(x+2h)-f(x+3h) \geq 0$$
Is it true that $$\forall (a,b) \in\mathbb R^2, |f(a)-f(b)|\leq \max\left(\left|f'\left(\dfrac{a+b} 2\right)\right|,\dfrac {|f'(a)+f'(b)|}{2}\right)\times |a-b|$$
1 reply
Dattier
Apr 29, 2025
Dattier
4 hours ago
< NA'T = < ADT wanted, starting with a right triangle, symmetric, projections
parmenides51   3
N 4 hours ago by tilya_TASh
Source: JBMO Shortlist 2018 G2
Let $ABC$ be a right angled triangle with $\angle A = 90^o$ and $AD$ its altitude. We draw parallel lines from $D$ to the vertical sides of the triangle and we call $E, Z$ their points of intersection with $AB$ and $AC$ respectively. The parallel line from $C$ to $EZ$ intersects the line $AB$ at the point $N$. Let $A' $ be the symmetric of $A$ with respect to the line $EZ$ and $I, K$ the projections of $A'$ onto $AB$ and $AC$ respectively. If $T$ is the point of intersection of the lines $IK$ and $DE$, prove that $\angle NA'T = \angle  ADT$.
3 replies
parmenides51
Jul 22, 2019
tilya_TASh
4 hours ago
Equivalent condition of the uniformly continuous fo a function
Alphaamss   0
4 hours ago
Source: Personal
Let $f_{a,b}(x)=x^a\cos(x^b),x\in(0,\infty)$. Get all the $(a,b)\in\mathbb R^2$ such that $f_{a,b}$ is uniformly continuous on $(0,\infty)$.
0 replies
Alphaamss
4 hours ago
0 replies
x(x - y) = 8y - 7 in NxN
parmenides51   4
N 4 hours ago by Namisgood
Source: JBMO 2008 Shortlist N1
Find all the positive integers $x$ and $y$ that satisfy the equation $x(x - y) = 8y - 7$
4 replies
parmenides51
Oct 14, 2017
Namisgood
4 hours ago
Two times derivable real function
Valentin Vornicu   13
N Apr 24, 2025 by solyaris
Source: RMO 2008, 11th Grade, Problem 3
Let $ f: \mathbb R \to \mathbb R$ be a function, two times derivable on $ \mathbb R$ for which there exist $ c\in\mathbb R$ such that
\[ \frac { f(b)-f(a) }{b-a} \neq f'(c) ,\] for all $ a\neq b \in \mathbb R$.

Prove that $ f''(c)=0$.
13 replies
Valentin Vornicu
Apr 30, 2008
solyaris
Apr 24, 2025
Two times derivable real function
G H J
Source: RMO 2008, 11th Grade, Problem 3
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Valentin Vornicu
7301 posts
#1 • 2 Y
Y by Adventure10, Mango247
Let $ f: \mathbb R \to \mathbb R$ be a function, two times derivable on $ \mathbb R$ for which there exist $ c\in\mathbb R$ such that
\[ \frac { f(b)-f(a) }{b-a} \neq f'(c) ,\] for all $ a\neq b \in \mathbb R$.

Prove that $ f''(c)=0$.
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harazi
5526 posts
#2 • 2 Y
Y by Adventure10, Mango247
The image of the function $ g(a,b)=\frac{f(a)-f(b)}{a-b}$ defined for $ a\ne b$ being an interval (connected subset of the real line) and $ f'(c)$ not being in this image, it follows that we may assume that $ f'(c)<g(a,b)$ for all $ a\ne b$. But then $ f'(c)\leq f'(x)$ for all $ x$ and so $ c$ is a minimum point of $ f'$. Of course, this can be written in 11-th grade vocabulary. :D
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Svejk
663 posts
#3 • 1 Y
Y by Adventure10
I tried to solve it harazi's way but I didn't get maximum since I didn't manage to prove that $ g(a,b)-f'(c)$ has the same sign for all $ a,b$.Can you be give me more detailes please :lol: ?The official solution is based on the fact that the function $ g(x)=f(x)-f'(c)\cdot x$ is injective ,hence monotonic.
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harazi
5526 posts
#4 • 1 Y
Y by Adventure10
The set of pairs $ (a,b)$ such that $ a\ne b$ is a connected subset of the plane and the function $ g$ is continuous on this domain, thus its image is a connected subset of the line, thus an interval. I agree however that this is not a solution of an 11-th grade student, but that's how life is. :D I will not be amazed if in a few years I see complex analysis, Lebesgue integration and other such stuff at RMO. It's quite à pity, it gives huges advantages to some people. :(
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enescu
741 posts
#5 • 2 Y
Y by Adventure10, Mango247
harazi wrote:
But then $ f'(c)\leq f'(x)$ for all $ x$ and so $ c$ is a minimum point of $ f'$.
Why for all $ x$?
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harazi
5526 posts
#6 • 2 Y
Y by Adventure10, Mango247
Well, $ f'(c)\leq g(a,x)$ for all $ a\ne x$ and now make $ a$ close to $ x$ and use the definition of derivative.
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harazi
5526 posts
#7 • 1 Y
Y by Adventure10
Well, I said however a very stupid thing: the function $ g$ should be defined on the set of pairs $ (a,b)$ such that $ a<b$ to ensure that its domain is connected. Of course, all the rest works with this modification, I don't know how I could write such a stupid thing. :D
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enescu
741 posts
#8 • 4 Y
Y by adityaguharoy, Adventure10, Mango247, RobertRogo
Well, when I created this problem, I was thinking to the obvious geometric meaning: if $ f''(c) \ne 0$, then $ f$ is strictly concave up or down on some neighbourhood of the point $ c$, thus one can draw a close enough parallel to the line tangent at $ c$ to the function's graph that intersects the graph in two points $ (a,f(a))$ and $ (b,f(b))$. The slope of that tangent would be $ \frac{f(b)-f(a)}{b-a}$, equal to the slope of the tangent at $ c$, that is,$ f'(c)$.
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subham1729
1479 posts
#9 • 1 Y
Y by Adventure10
One of above solutions uses that $C=\mathbb{R}^2-\{(a,a) \mid a \in \mathbb{R}\}$ is connected, but why $C$ is connected ? $C$ has clearly two connected components. However with this spirit we can also solve the problem, extend $g(a,b)=\frac{f(a)-f(b)}{a-b}$ to whole plane defining $g(a,a)=f'(a)$ and now $g$ is continuous on whole plane and do similar thing.
This post has been edited 1 time. Last edited by subham1729, Jun 14, 2016, 4:53 AM
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Raunii
28 posts
#10
Y by
Svejk wrote:
I tried to solve it harazi's way but I didn't get maximum since I didn't manage to prove that $ g(a,b)-f'(c)$ has the same sign for all $ a,b$.Can you be give me more detailes please :lol: ?The official solution is based on the fact that the function $ g(x)=f(x)-f'(c)\cdot x$ is injective ,hence monotonic.

where did you find the official solution?
This post has been edited 1 time. Last edited by Raunii, Mar 15, 2020, 6:05 PM
Reason: .
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Rohit-2006
237 posts
#11
Y by
Too easy for grade 11....
Attachments:
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solyaris
632 posts
#12
Y by
@above: This is not a valid argument. From the MVT you only get for every $(a,b)$ there exists an $m$ with the desired property. So you get $f'(m) \neq f'(c)$ only for values $m$ in some set $M$, which has to be dense in the reals, but $M$ need not be an interval, so the IVP you use later on in your proof does not give a contradiction. (See the solutions above for proofs that avoid this problem.)
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Rohit-2006
237 posts
#13
Y by
Solyaris....can you please elaborate what you are trying to say....I can't get it what you are trying to say....I am just interested that $f'$ is continuous on $\mathbb{R}$ and that is true because $f$ is twice differentiable.
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solyaris
632 posts
#15
Y by
@above. To elaborate: Let $M = \{x \in R : f'(x) \neq f'(c)\}$. In the first paragraph you show that for all $a < b$ $M$ has to contain some $m \in (a,b)$ (which means that $M$ is a dense subset of the real numbers). This part of your proof is fine. But in order to make the proof of the green claim work you need to show that $M$ contains all real numbers. This is missing in your proof (if I interpret you proof correctly).
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