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k a June Highlights and 2025 AoPS Online Class Information
jlacosta   0
Jun 2, 2025
Congratulations to all the mathletes who competed at National MATHCOUNTS! If you missed the exciting Countdown Round, you can watch the video at this link. Are you interested in training for MATHCOUNTS or AMC 10 contests? How would you like to train for these math competitions in half the time? We have accelerated sections which meet twice per week instead of once starting on July 8th (7:30pm ET). These sections fill quickly so enroll today!

[list][*]MATHCOUNTS/AMC 8 Basics
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[*]AMC 10 Problem Series[/list]
For those interested in Olympiad level training in math, computer science, physics, and chemistry, be sure to enroll in our WOOT courses before August 19th to take advantage of early bird pricing!

Summer camps are starting this 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 a transformative 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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0 replies
jlacosta
Jun 2, 2025
0 replies
Iranian tough nut: AA', BN, CM concur in Gergonne picture
grobber   69
N 5 minutes ago by zuat.e
Source: Iranian olympiad/round 3/2002
Let $ABC$ be a triangle. The incircle of triangle $ABC$ touches the side $BC$ at $A^{\prime}$, and the line $AA^{\prime}$ meets the incircle again at a point $P$. Let the lines $CP$ and $BP$ meet the incircle of triangle $ABC$ again at $N$ and $M$, respectively. Prove that the lines $AA^{\prime}$, $BN$ and $CM$ are concurrent.
69 replies
grobber
Dec 29, 2003
zuat.e
5 minutes ago
Inspired by current year (2025)
Rijul saini   4
N 21 minutes ago by Rg230403
Source: India IMOTC 2025 Day 4 Problem 1
Let $k>2$ be an integer. We call a pair of integers $(a,b)$ $k-$good if \[0\leqslant a<k,\hspace{0.2cm} 0<b \hspace{1cm} \text{and} \hspace{1cm} (a+b)^2=ka+b\]Prove that the number of $k-$good pairs is a power of $2$.

Proposed by Prithwijit De and Rohan Goyal
4 replies
Rijul saini
Yesterday at 6:46 PM
Rg230403
21 minutes ago
Write down sum or product of two numbers
Rijul saini   2
N 33 minutes ago by Rg230403
Source: India IMOTC Practice Test 2 Problem 3
Suppose Alice's grimoire has the number $1$ written on the first page and $n$ empty pages. Suppose in each of the next $n$ seconds, Alice can flip to the next page, and write down the sum or product of two numbers (possibly the same) which are already written in her grimoire.

Let $F(n)$ be the largest possible number such that for any $k < F(n)$, Alice can write down the number $k$ on the last page of her grimoire. Prove that there exists a positive integer $N$ such that for all $n>N$, we have that \[n^{0.99n}\leqslant F(n)\leqslant n^{1.01n}.\]
Proposed by Rohan Goyal and Pranjal Srivastava
2 replies
1 viewing
Rijul saini
Yesterday at 6:56 PM
Rg230403
33 minutes ago
"all of the stupid geo gets sent to tst 2/5" -allen wang
pikapika007   27
N an hour ago by HamstPan38825
Source: USA TST 2024/2
Let $ABC$ be a triangle with incenter $I$. Let segment $AI$ intersect the incircle of triangle $ABC$ at point $D$. Suppose that line $BD$ is perpendicular to line $AC$. Let $P$ be a point such that $\angle BPA = \angle PAI = 90^\circ$. Point $Q$ lies on segment $BD$ such that the circumcircle of triangle $ABQ$ is tangent to line $BI$. Point $X$ lies on line $PQ$ such that $\angle IAX = \angle XAC$. Prove that $\angle AXP = 45^\circ$.

Luke Robitaille
27 replies
+1 w
pikapika007
Dec 11, 2023
HamstPan38825
an hour ago
Another integral limit
RobertRogo   3
N Today at 11:59 AM by Bayram_Turayew
Source: "Traian Lalescu" student contest 2025, Section A, Problem 3
Let $f \colon [0, \infty) \to \mathbb{R}$ be a function differentiable at 0 with $f(0) = 0$. Find
$$\lim_{n \to \infty} \frac{1}{n} \int_{2^n}^{2^{n+1}} f\left(\frac{\ln x}{x}\right) dx$$
3 replies
RobertRogo
May 9, 2025
Bayram_Turayew
Today at 11:59 AM
Derivatives on a Functional Equation
Kingofmath101   1
N Today at 11:28 AM by Mathzeus1024
Let $g$ be a smooth function on $\mathbb{R}$ where $g^{(n)}(0)$ for all $n \in \mathbb{N}^+$ and $g$ satisfies

$$g(x) = xg(x^2 - 4)$$
for all $x \in \mathbb{R}$. Prove that $g^{(n)}(-2) = g^{(n)}(2) = 0$ for all $n \in \mathbb{N}$.

1 reply
Kingofmath101
Jun 4, 2017
Mathzeus1024
Today at 11:28 AM
Infinite series with Pell numbers
Entrepreneur   12
N Today at 11:27 AM by Entrepreneur
Source: Own
Evaluate the sum $$\color{blue}{\sum_{k=1}^\infty\frac{P_kx^k}{k!}.}$$Where $P_n$ denotes the n-th Pell Number given by $P_0=0,P_1=1$ and $$P_{n+2}=2P_{n+1}+P_n.$$
12 replies
Entrepreneur
Nov 5, 2024
Entrepreneur
Today at 11:27 AM
3xn matrice with combinatorical property
Sebaj71Tobias   2
N Today at 10:49 AM by c00lb0y
Let"s have a 3xn matrice with the following properties:
The firs row of the matrice is 1,2,3,... ,n in this order.
The second and the third rows are permutations of the first.
Very important, that in each column thera are different entries.
How many matrices with thees properties are there?

The answer for 2xn matrices is well-known, but what is the answer for 3xn, or for kxn ( k<=n) ?
2 replies
Sebaj71Tobias
Jun 1, 2025
c00lb0y
Today at 10:49 AM
The matrix in some degree is a scalar
FFA21   5
N Today at 10:09 AM by c00lb0y
Source: MSU algebra olympiad 2025 P2
$A\in M_{3\times 3}$ invertible, for an infinite number of $k$:
$tr(A^k)=0$
Is it true that $\exists n$ such that $A^n$ is a scalar
5 replies
FFA21
May 20, 2025
c00lb0y
Today at 10:09 AM
Approximate the integral
ILOVEMYFAMILY   2
N Today at 9:39 AM by ILOVEMYFAMILY
Approximate the integral
\[
I = \int_0^1 \frac{(2^x + 2)\, dx}{1 + x^4}
\]using the trapezoidal rule with accuracy $10^{-2}$.
2 replies
ILOVEMYFAMILY
Today at 5:27 AM
ILOVEMYFAMILY
Today at 9:39 AM
Approximate the integral
ILOVEMYFAMILY   0
Today at 5:26 AM
Approximate the integral
\[
I = \int_0^1 \frac{(2x^2 + 1)\, dx}{\sqrt{(1 + x^2)(2 - x^2)}}
\]using the parabola method (Simpson's rule) with accuracy $\varepsilon = 10^{-4}$.
0 replies
ILOVEMYFAMILY
Today at 5:26 AM
0 replies
ISI UGB 2025
Entrepreneur   5
N Today at 1:37 AM by Gauler
Source: ISI UGB 2025
1.)
Suppose $f:\mathbb R\to\mathbb R$ is differentiable and $|f'(x)|<\frac 12\;\forall\;x\in\mathbb R.$ Show that for some $x_0\in\mathbb R,f(x_0)=x_0.$

3.)
Suppose $f:[0,1]\to\mathbb R$ is differentiable with $f(0)=0.$ If $|f'(x)|\le f(x)\;\forall\;x\in[0,1],$ then show that $f(x)=0\;\forall\;x.$

4.)
Let $S^1=\{z\in\mathbb C:|z|=1\}$ be the unit circle in the complex plane. Let $f:S^1\to S^1$ be the map given by $f(z)=z^2.$ We define $f^{(1)}:=f$ and $f^{(k+1)}=f\circ f^{(k)}$ for $k\ge 1.$ The smallest positive integer $n$ such that $f^n(z)=z$ is called period of $z.$ Determine the total number of points $S^1$ of period $2025.$

6.)
Let $\mathbb N$ denote the set of natural numbers, and let $(a_i,b_i), 1\le i\le 9,$ be nine distinct tuples in $\mathbb N\times\mathbb N.$ Show that there are $3$ distinct elements in the set $\{2^{a_i}3^{b_i}:1\le i\le 9\}$ whose product is a perfect cube.

8.)
Let $n\ge 2$ and let $a_1\le a_2\le\cdots\le a_n$ be positive integers such that $$\sum_{i=1}^n a_i=\prod_{i=1}^n a_i.$$Prove that $$\sum_{i=1}^n a_i\le 2n$$and determine when equality holds.
5 replies
Entrepreneur
May 27, 2025
Gauler
Today at 1:37 AM
sequence of highly correlated rv pairs
Konigsberg   1
N Yesterday at 10:57 PM by yofro
For a given $0 < \varepsilon< 1$, construct a sequence of random variables $X_1,\dots,X_n$ such that
$$
\operatorname{Corr}(X_i,X_{i+1}) = 1-\varepsilon,\qquad 1\le i\le n-1.
$$Let
$$
f(\varepsilon)=\min\left\{\,n\in\mathbb N \mid \text{it is possible that }\operatorname{Corr}(X_1,X_n)<0\right\}.
$$Find positive constants $c_1$ and $c_2$ such that
$$
\lim_{\varepsilon\to0}\frac{f(\varepsilon)}{c_1\,\varepsilon^{c_2}}=1.
$$
1 reply
Konigsberg
May 23, 2025
yofro
Yesterday at 10:57 PM
functional analysis
ILOVEMYFAMILY   1
N Yesterday at 2:27 PM by alexheinis
Let $E$, $F$ be normed spaces with $E$ a Banach space. Suppose $\{A_n: E \to F\}$ is a family of continuous linear maps. Prove that the set
\[
X = \left\{ x \in E \mid \sup_{n\geq 1}|| A_n(x)||< +\infty  \right\}
\]is either of first category in $E$ or is equal to the whole space $E$.
1 reply
ILOVEMYFAMILY
Jun 2, 2025
alexheinis
Yesterday at 2:27 PM
Determining Integers From Sums
oVlad   2
N Apr 14, 2025 by oVlad
Source: Romania Junior TST 2025 Day 1 P3
Let $n\geqslant 3$ be a positiv integer. Ana chooses the positive integers $a_1,a_2,\ldots,a_n$ and for any non-empty subset $A\subseteq\{1,2,\ldots,n\}$ she computes the sum \[s_A=\sum_{k
\in A}a_k.\]She orders these sums $s_1\leqslant s_2\leqslant\cdots\leqslant s_{2^n-1}.$ Prove that there exists a subset $B\subseteq\{1,2,\ldots,2^n-1\}$ with $2^{n-2}+1$ elements such that, regardless of the integers $a_1,a_2,\ldots,a_n$ chosen by Ana, these can be determined by only knowing the sums $s_i$ with $i\in B.$
2 replies
oVlad
Apr 12, 2025
oVlad
Apr 14, 2025
Determining Integers From Sums
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G H BBookmark kLocked kLocked NReply
Source: Romania Junior TST 2025 Day 1 P3
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oVlad
1746 posts
#1 • 1 Y
Y by anantmudgal09
Let $n\geqslant 3$ be a positiv integer. Ana chooses the positive integers $a_1,a_2,\ldots,a_n$ and for any non-empty subset $A\subseteq\{1,2,\ldots,n\}$ she computes the sum \[s_A=\sum_{k
\in A}a_k.\]She orders these sums $s_1\leqslant s_2\leqslant\cdots\leqslant s_{2^n-1}.$ Prove that there exists a subset $B\subseteq\{1,2,\ldots,2^n-1\}$ with $2^{n-2}+1$ elements such that, regardless of the integers $a_1,a_2,\ldots,a_n$ chosen by Ana, these can be determined by only knowing the sums $s_i$ with $i\in B.$
This post has been edited 1 time. Last edited by oVlad, Apr 12, 2025, 9:45 AM
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removablesingularity
569 posts
#2
Y by
I don't know if I misunderstood the problem, but if $A = \{i\},i=1,2,\cdots,n$ and $B$ contains $A$ does it solve the problem?Since it contains the single element?
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oVlad
1746 posts
#3
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removablesingularity wrote:
I don't know if I misunderstood the problem, but if $A = \{i\},i=1,2,\cdots,n$ and $B$ contains $A$ does it solve the problem?Since it contains the single element?
The problem is that $s_i{}$ denotes the $i$-th smallest sum, not necessarily the sum of the singleton $\{a_i\}$. For instance, if $a_1{}$ and $a_2{}$ are rather small, but all other $a_i$'s are much much larger, then $s_3=a_1+a_2$ not $a_3{}$ so it's useless information.
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