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k a April Highlights and 2025 AoPS Online Class Information
jlacosta   0
Wednesday at 3:18 PM
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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0 replies
jlacosta
Wednesday at 3:18 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
An interesting limit
Alphaamss   1
N 38 minutes ago by Snoop76
Suppose $$x_1=\frac\pi2,\quad x_{n+1}=x_n-\frac{\sin x_n}{n+1},$$I can prove that the sequence $\{nx_n\}$ is convergent by monotone bounded convergence theorem.
Is there any method to compute the limit of $\{nx_n\}$, or give the asymptotic representation of $\{nx_n\}$? Any help and hints will welcome!
1 reply
Alphaamss
Yesterday at 9:16 AM
Snoop76
38 minutes ago
inequality ( 4 var
SunnyEvan   2
N an hour ago by ektorasmiliotis
Let $ a,b,c,d \in R $ , such that $ a+b+c+d=4 . $ Prove that :
$$ a^4+b^4+c^4+d^4+3 \geq \frac{7}{4}(a^3+b^3+c^3+d^3) $$$$ a^4+b^4+c^4+d^4+ \frac{252}{25} \geq \frac{88}{25}(a^3+b^3+c^3+d^3) $$equality cases : ?
2 replies
SunnyEvan
6 hours ago
ektorasmiliotis
an hour ago
Inspired by JK1603JK
sqing   6
N an hour ago by sqing
Source: Own
Let $ a,b,c\geq 0 $ and $ab+bc+ca=1.$ Prove that$$\frac{abc-2}{abc-1}\ge \frac{4(a^2b+b^2c+c^2a)}{a^3b+b^3c+c^3a+1} $$
6 replies
sqing
Today at 3:31 AM
sqing
an hour ago
Basis vectors type question
RenheMiResembleRice   4
N 2 hours ago by solyaris
Source: Nuohan Ju, Jiaoyuan Gong
Solve the following attached with explanation.
4 replies
RenheMiResembleRice
Yesterday at 2:51 AM
solyaris
2 hours ago
Range of solutions to the log equation
obihs   3
N 2 hours ago by solyaris
Source: Own
Let $n$ be a positive integer, and consider the equation:
$$(\log x)^n - x + 1 = 0\quad\cdots(\heartsuit)$$Answer the following questions. You may assume that $2.7<e<2.72$ is known.

$(1)\quad$ Determine the number of real solutions of equation $(\heartsuit)$ for each $n$.

$(2)\quad$ For $n\ge 3$ , let $r_n$ be the segond largest real solution of $(\heartsuit)$.

$(\i)\quad$ Find $\alpha$ such that $\lim_{n\to\infty} r_n =\alpha.$

$(\i\i)\quad$ Find $\lfloor\beta\rfloor$, where $\beta$ is defined as

$$\lim_{n\to\infty}n(r_n-\alpha)=\beta.$$
3 replies
obihs
Apr 1, 2025
solyaris
2 hours ago
Geometry problem
kjhgyuio   1
N 2 hours ago by Mathzeus1024
Source: smo
In trapezium ABCD,AD is parallel to BC and points E and F are midpoints of AB and DC respectively. If
Area of AEFD/Area of EBCF =√3 + 1/3-√3 and the area of triangle ABD is √3 .find the area of trapezium ABCD
1 reply
kjhgyuio
Apr 1, 2025
Mathzeus1024
2 hours ago
D1018 : Can you do that ?
Dattier   1
N 2 hours ago by Dattier
Source: les dattes à Dattier
We can find $A,B,C$, such that $\gcd(A,B)=\gcd(C,A)=\gcd(A,2)=1$ and $$\forall n \in \mathbb N^*, (C^n \times B \mod A) \mod 2=0 $$.

For example :

$C=20$
$A=47650065401584409637777147310342834508082136874940478469495402430677786194142956609253842997905945723173497630499054266092849839$

$B=238877301561986449355077953728734922992395532218802882582141073061059783672634737309722816649187007910722185635031285098751698$

Can you find $A,B,C$ such that $A>3$ is prime, $C,B \in (\mathbb Z/A\mathbb Z)^*$ with $o(C)=(A-1)/2$ and $$\forall n \in \mathbb N^*, (C^n \times B \mod A) \mod 2=0 $$?
1 reply
Dattier
Mar 24, 2025
Dattier
2 hours ago
2025 Caucasus MO Juniors P3
BR1F1SZ   1
N 2 hours ago by FarrukhKhayitboyev
Source: Caucasus MO
Let $K$ be a positive integer. Egor has $100$ cards with the number $2$ written on them, and $100$ cards with the number $3$ written on them. Egor wants to paint each card red or blue so that no subset of cards of the same color has the sum of the numbers equal to $K$. Find the greatest $K$ such that Egor will not be able to paint the cards in such a way.
1 reply
BR1F1SZ
Mar 26, 2025
FarrukhKhayitboyev
2 hours ago
1 area = 2025 points
giangtruong13   0
2 hours ago
In a plane give a set $H$ that has 8097 distinct points with area of a triangle that has 3 points belong to $H$ all $ \leq 1$. Prove that there exists a triangle $G$ that has the area $\leq 1 $ contains at least 2025 points that belong to $H$( each of that 2025 points can be inside the triangle or lie on the edge of triangle $G$)X
0 replies
giangtruong13
2 hours ago
0 replies
Burak0609
Burak0609   0
2 hours ago
So if $2 \nmid n\implies$ $2d_2+d_4+d_5=d_7$ is even it's contradiction. I mean $2 \mid n and d_2=2$.
If $3\mid n \implies d_3=3$ and $(d_6+d_7)^2=n+1,3d_6d_7=n \implies d_6^2-d_6d_7+d_7^2=1$,we can see the only solution is$d_6=d_7=1$ and it is contradiction.
If $4 \mid n d_3=4$ and $(d_6+d_7)^2-4d_6d_7=1 \implies d_7=d_6+1$. So $n=4d_6(d_6+1)$.İt means $8 \mid n$.
If $d_6=8 n=4.8.9=288$ but $3 \nmid n$.İt is contradiction.
If $d_5=8$ we have 2 option. Firstly $d_4=5 \implies 2d_2+d_4+d_5=17=d_7 d_6=16$ but $10 \mid n$ is contradiction. Secondly $d_4=7 \implies d_7=2.2+7+8=19 and d_6=18$ but $3 \nmid n$ is contradiction. I mean $d_4=8 \implies d_7=d_5+12, n=4(d_5+11)(d_5+12) and d_5 \mid n=4(d_5+11)(d_5+12)$. So $d_5 \mid 4.11.12 \implies d_5 \mid 16.11$. If $d_5=16 d_6=27$ but $3 \nmid n$ is contradiction. I mean $d_5=11,d_6=22,d_7=23$. The only solution is $n=2024$.
0 replies
Burak0609
2 hours ago
0 replies
Putnam 1979 B5
ZETA_in_olympiad   2
N 3 hours ago by Mathzeus1024
In the plane, let $C$ be a closed convex set that contains $(0,0)$ but no other point with integer coordinates. Suppose that $A(C)$, the area of $C$, is equally distributed among the four quadrants. Prove that $A(C) \leq 4.$
2 replies
ZETA_in_olympiad
Apr 8, 2022
Mathzeus1024
3 hours ago
Good Partitions
va2010   25
N 3 hours ago by lelouchvigeo
Source: 2015 ISL C3
For a finite set $A$ of positive integers, a partition of $A$ into two disjoint nonempty subsets $A_1$ and $A_2$ is $\textit{good}$ if the least common multiple of the elements in $A_1$ is equal to the greatest common divisor of the elements in $A_2$. Determine the minimum value of $n$ such that there exists a set of $n$ positive integers with exactly $2015$ good partitions.
25 replies
va2010
Jul 7, 2016
lelouchvigeo
3 hours ago
An inequality on triangles sides
nAalniaOMliO   7
N 4 hours ago by navier3072
Source: Belarusian National Olympiad 2025
Numbers $a,b,c$ are lengths of sides of some triangle. Prove the inequality$$\frac{a}{b+c-a}+\frac{b}{c+a-b}+\frac{c}{a+b-c} \geq \frac{a+b}{2c}+\frac{b+c}{2a}+\frac{c+a}{2b}$$
7 replies
nAalniaOMliO
Mar 28, 2025
navier3072
4 hours ago
D is incenter
Layaliya   3
N 4 hours ago by rong2020
Source: From my friend in Indonesia
Given an acute triangle \( ABC \) where \( AB > AC \). Point \( O \) is the circumcenter of triangle \( ABC \), and \( P \) is the projection of point \( A \) onto line \( BC \). The midpoints of \( BC \), \( CA \), and \( AB \) are \( D \), \( E \), and \( F \), respectively. The line \( AO \) intersects \( DE \) and \( DF \) at points \( Q \) and \( R \), respectively. Prove that \( D \) is the incenter of triangle \( PQR \).
3 replies
Layaliya
Yesterday at 11:03 AM
rong2020
4 hours ago
CDF of normal distribution
We2592   2
N Mar 29, 2025 by rchokler
Q) We know that the PDF of normal distribution of $X$ id defined by
\[
f(x) = \frac{1}{\sqrt{2\pi} \sigma} e^{-\frac{(x-\mu)^2}{2\sigma^2}}
\]now what is CDF or cumulative distribution function $F_X(x)=P[X\leq x]$

how to integrate ${-\infty} \to x$
please help
2 replies
We2592
Mar 29, 2025
rchokler
Mar 29, 2025
CDF of normal distribution
G H J
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We2592
135 posts
#1
Y by
Q) We know that the PDF of normal distribution of $X$ id defined by
\[
f(x) = \frac{1}{\sqrt{2\pi} \sigma} e^{-\frac{(x-\mu)^2}{2\sigma^2}}
\]now what is CDF or cumulative distribution function $F_X(x)=P[X\leq x]$

how to integrate ${-\infty} \to x$
please help
This post has been edited 1 time. Last edited by We2592, Mar 29, 2025, 2:09 AM
Reason: error
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Tricky123
37 posts
#2
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Please help...
Z K Y
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rchokler
2949 posts
#3 • 1 Y
Y by MS_asdfgzxcvb
You can do it in terms of the error function $\operatorname{erf}(x)=\frac{2}{\sqrt{\pi}}\int_0^xe^{-t^2}\ dt$.

Use the substitution $u=\frac{t-\mu}{\sigma\sqrt{2}}$, $du=\frac{dt}{\sigma\sqrt{2}}$.

$F(x)=\int_{-\infty}^x\frac{1}{\sqrt{2\pi} \sigma} e^{-\frac{(t-\mu)^2}{2\sigma^2}}\ dt=\frac{1}{\sqrt{\pi}}\int_{-\infty}^{\frac{x-\mu}{\sigma\sqrt{2}}}e^{-u^2}\ du=\frac{1}{2}\operatorname{erf}\left(\frac{x-\mu}{\sigma\sqrt{2}}\right)-\lim_{u\to-\infty}\frac{1}{2}\operatorname{erf}(u)=\frac{1}{2}+\frac{1}{2}\operatorname{erf}\left(\frac{x-\mu}{\sigma\sqrt{2}}\right)$.

Note that this is $F(x)=\Phi\left(\frac{x-\mu}{\sigma}\right)$ where $\Phi(z)$ is the CDF of the standard normal distribution.
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