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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.

WOOT early bird pricing is in effect, don’t miss out! If you took MathWOOT Level 2 last year, no worries, it is all new problems this year! Our Worldwide Online Olympiad Training program is for high school level competitors. AoPS designed these courses to help our top students get the deep focus they need to succeed in their specific competition goals. Check out the details at this link for all our WOOT programs in math, computer science, chemistry, and physics.

Looking for summer camps in math and language arts? Be sure to check out the video-based summer camps offered at the Virtual Campus that are 2- to 4-weeks in duration. 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)!

Be sure to mark your calendars for the following events:
[list][*]April 3rd (Webinar), 4pm PT/7:00pm ET, Learning with AoPS: Perspectives from a Parent, Math Camp Instructor, and University Professor
[*]April 8th (Math Jam), 4:30pm PT/7:30pm ET, 2025 MATHCOUNTS State Discussion
April 9th (Webinar), 4:00pm PT/7:00pm ET, Learn about Video-based Summer Camps at the Virtual Campus
[*]April 10th (Math Jam), 4:30pm PT/7:30pm ET, 2025 MathILy and MathILy-Er Math Jam: Multibackwards Numbers
[*]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
IMO Shortlist 2011, G4
WakeUp   125
N 8 minutes ago by Davdav1232
Source: IMO Shortlist 2011, G4
Let $ABC$ be an acute triangle with circumcircle $\Omega$. Let $B_0$ be the midpoint of $AC$ and let $C_0$ be the midpoint of $AB$. Let $D$ be the foot of the altitude from $A$ and let $G$ be the centroid of the triangle $ABC$. Let $\omega$ be a circle through $B_0$ and $C_0$ that is tangent to the circle $\Omega$ at a point $X\not= A$. Prove that the points $D,G$ and $X$ are collinear.

Proposed by Ismail Isaev and Mikhail Isaev, Russia
125 replies
WakeUp
Jul 13, 2012
Davdav1232
8 minutes ago
Z[x], P(\sqrt[3]5+\sqrt[3]25)=5+\sqrt[3]5
jasperE3   5
N 27 minutes ago by Assassino9931
Source: VJIMC 2013 2.3
Prove that there is no polynomial $P$ with integer coefficients such that $P\left(\sqrt[3]5+\sqrt[3]{25}\right)=5+\sqrt[3]5$.
5 replies
1 viewing
jasperE3
May 31, 2021
Assassino9931
27 minutes ago
IMO problem 1
iandrei   77
N 27 minutes ago by YaoAOPS
Source: IMO ShortList 2003, combinatorics problem 1
Let $A$ be a $101$-element subset of the set $S=\{1,2,\ldots,1000000\}$. Prove that there exist numbers $t_1$, $t_2, \ldots, t_{100}$ in $S$ such that the sets \[ A_j=\{x+t_j\mid x\in A\},\qquad j=1,2,\ldots,100  \] are pairwise disjoint.
77 replies
1 viewing
iandrei
Jul 14, 2003
YaoAOPS
27 minutes ago
Divisibility on 101 integers
BR1F1SZ   4
N an hour ago by BR1F1SZ
Source: Argentina Cono Sur TST 2024 P2
There are $101$ positive integers $a_1, a_2, \ldots, a_{101}$ such that for every index $i$, with $1 \leqslant i \leqslant 101$, $a_i+1$ is a multiple of $a_{i+1}$. Determine the greatest possible value of the largest of the $101$ numbers.
4 replies
BR1F1SZ
Aug 9, 2024
BR1F1SZ
an hour ago
Putnam 1972 A2
sqrtX   2
N 4 hours ago by KAME06
Source: Putnam 1972
Let $S$ be a set with a binary operation $\ast$ such that
1) $a \ast(a\ast b)=b$ for all $a,b\in S$.
2) $(a\ast b)\ast b=a$ for all $a,b\in S$.
Show that $\ast$ is commutative and give an example where $\ast$ is not associative.
2 replies
sqrtX
Feb 17, 2022
KAME06
4 hours ago
Limit with sin^2x
Quantum_fluctuations   7
N Today at 7:25 AM by P162008

Evaluate:

$\lim_{x \to 0} \left( 1^{1/\sin^2 x} + 2^{1/\sin^2 x} + 3^{1/\sin^2 x} + .  .  . + n^{1/\sin^2 x} \right)^{\sin^2 x}$
7 replies
Quantum_fluctuations
Apr 26, 2020
P162008
Today at 7:25 AM
Decimal number defined recursively by digit sums modulo 10
fermion13pi   2
N Today at 7:20 AM by solyaris
Source: Competição Elon Lages Lima
Consider the real number written in decimal notation:
r = 0.235831...
where, starting from the third digit after the decimal point, each digit is equal to the remainder when the sum of the previous two digits is divided by 10.

Which of the following statements is true?

(a) (10⁶⁰ - 1).r is an integer
(b) (10²⁵ - 1).r is an integer
(c) (10¹⁷ - 1).r is an integer
(d) r is an irrational algebraic number
(e) r is an irrational transcendental number

(Recall that a complex number is called algebraic if it is a root of a non-zero polynomial with integer coefficients.)
2 replies
fermion13pi
Yesterday at 11:14 PM
solyaris
Today at 7:20 AM
Integrable function: + and - on every subinterval.
SPQ   3
N Today at 7:06 AM by solyaris
Provide a function integrable on [a, b] such that f takes on positive and negative values on every subinterval (c, d) of [a, b]. Prove your function satisfies both conditions.
3 replies
SPQ
Today at 2:40 AM
solyaris
Today at 7:06 AM
Putnam 1999 A4
djmathman   7
N Today at 7:05 AM by P162008
Sum the series \[\sum_{m=1}^\infty\sum_{n=1}^\infty\dfrac{m^2n}{3^m(n3^m+m3^n)}.\]
7 replies
djmathman
Dec 22, 2012
P162008
Today at 7:05 AM
Find the greatest possible value of the expression
BEHZOD_UZ   1
N Today at 6:34 AM by alexheinis
Source: Yandex Uzbekistan Coding and Math Contest 2025
Let $a, b, c, d$ be complex numbers with $|a| \le 1, |b| \le 1, |c| \le 1, |d| \le 1$. Find the greatest possible value of the expression $$|ac+ad+bc-bd|.$$
1 reply
BEHZOD_UZ
Yesterday at 11:56 AM
alexheinis
Today at 6:34 AM
Problem with lcm
snowhite   2
N Today at 6:21 AM by snowhite
Prove that $\underset{n\to \infty }{\mathop{\lim }}\,\sqrt[n]{lcm(1,2,3,...,n)}=e$
Please help me! Thank you!
2 replies
snowhite
Today at 5:19 AM
snowhite
Today at 6:21 AM
combinatorics
Hello_Kitty   2
N Yesterday at 10:23 PM by Hello_Kitty
How many $100$ digit numbers are there
- not including the sequence $123$ ?
- not including the sequences $123$ and $231$ ?
2 replies
Hello_Kitty
Apr 17, 2025
Hello_Kitty
Yesterday at 10:23 PM
Sequence of functions
Squeeze   2
N Yesterday at 10:22 PM by Hello_Kitty
Q) let $f_n:[-1,1)\to\mathbb{R}$ and $f_n(x)=x^{n}$ then is this uniformly convergence on $(0,1)$ comment on uniformly convergence on $[0,1]$ where in general it is should be uniformly convergence.

My I am trying with some contradicton method like chose $\epsilon=1$ and trying to solve$|f_n(a)-f(a)|<\epsilon=1$
Next take a in (0,1) and chose a= 2^1/N but not solution
How to solve like this way help.
2 replies
Squeeze
Apr 18, 2025
Hello_Kitty
Yesterday at 10:22 PM
A in M2(prime), A=B^2 and det(B)=p^2
jasperE3   1
N Yesterday at 9:59 PM by KAME06
Source: VJIMC 2012 1.2
Determine all $2\times2$ integer matrices $A$ having the following properties:

$1.$ the entries of $A$ are (positive) prime numbers,
$2.$ there exists a $2\times2$ integer matrix $B$ such that $A=B^2$ and the determinant of $B$ is the square of a prime number.
1 reply
jasperE3
May 31, 2021
KAME06
Yesterday at 9:59 PM
locus of intersections of non parallels isosceles trapezoids
parmenides51   1
N Jul 3, 2024 by old_csk_mo
Source: Czech And Slovak Mathematical Olympiad, Round III, A 2010 p4
A circle $k$ is given with a non-diameter chord $AC$. On the tangent at point $A$ select point $X \ne A$ and mark $D$ the intersection of the circle $k$ with the interior of the line $XC$ (if any). Let $B$ a point in circle $k$ such that quadrilateral $ABCD$ is a trapezoid . Determine the set of intersections of lines $BC$ and $AD$ belonging to all such trapezoids.
1 reply
parmenides51
Jan 31, 2020
old_csk_mo
Jul 3, 2024
locus of intersections of non parallels isosceles trapezoids
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G H BBookmark kLocked kLocked NReply
Source: Czech And Slovak Mathematical Olympiad, Round III, A 2010 p4
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parmenides51
30630 posts
#1 • 1 Y
Y by Adventure10
A circle $k$ is given with a non-diameter chord $AC$. On the tangent at point $A$ select point $X \ne A$ and mark $D$ the intersection of the circle $k$ with the interior of the line $XC$ (if any). Let $B$ a point in circle $k$ such that quadrilateral $ABCD$ is a trapezoid . Determine the set of intersections of lines $BC$ and $AD$ belonging to all such trapezoids.
This post has been edited 1 time. Last edited by parmenides51, Jan 31, 2020, 11:24 AM
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old_csk_mo
102 posts
#2
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official solution: https://www.matematickaolympiada.cz/media/3459589/a59angl.pdf#page=17
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