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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
May 1, 2025
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.

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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
May 1, 2025
0 replies
GMO 2024 P1
Z4ADies   5
N 10 minutes ago by awesomeming327.
Source: Geometry Mains Olympiad (GMO) 2024 P1
Let \( ABC \) be an acute triangle. Define \( I \) as its incenter. Let \( D \) and \( E \) be the incircle's tangent points to \( AC \) and \( AB \), respectively. Let \( M \) be the midpoint of \( BC \). Let \( G \) be the intersection point of a perpendicular line passing through \( M \) to \( DE \). Line \( AM \) intersects the circumcircle of \( \triangle ABC \) at \( H \). The circumcircle of \( \triangle AGH \) intersects line \( GM \) at \( J \). Prove that quadrilateral \( BGCJ \) is cyclic.

Author:Ismayil Ismayilzada (Azerbaijan)
5 replies
Z4ADies
Oct 20, 2024
awesomeming327.
10 minutes ago
Power sequence
TheUltimate123   7
N 21 minutes ago by MathLuis
Source: ELMO Shortlist 2023 N2
Determine the greatest positive integer \(n\) for which there exists a sequence of distinct positive integers \(s_1\), \(s_2\), \(\ldots\), \(s_n\) satisfying \[s_1^{s_2}=s_2^{s_3}=\cdots=s_{n-1}^{s_n}.\]
Proposed by Holden Mui
7 replies
TheUltimate123
Jun 29, 2023
MathLuis
21 minutes ago
Interesting inequality of sequence
GeorgeRP   1
N 35 minutes ago by Assassino9931
Source: Bulgaria IMO TST 2025 P2
Let $d\geq 2$ be an integer and $a_0,a_1,\ldots$ is a sequence of real numbers for which $a_0=a_1=\cdots=a_d=1$ and:
$$a_{k+1}\geq a_k-\frac{a_{k-d}}{4d}, \forall_{k\geq d}$$Prove that all elements of the sequence are positive.
1 reply
GeorgeRP
Yesterday at 7:47 AM
Assassino9931
35 minutes ago
IMO Shortlist 2013, Combinatorics #4
lyukhson   21
N 2 hours ago by Ciobi_
Source: IMO Shortlist 2013, Combinatorics #4
Let $n$ be a positive integer, and let $A$ be a subset of $\{ 1,\cdots ,n\}$. An $A$-partition of $n$ into $k$ parts is a representation of n as a sum $n = a_1 + \cdots + a_k$, where the parts $a_1 , \cdots , a_k $ belong to $A$ and are not necessarily distinct. The number of different parts in such a partition is the number of (distinct) elements in the set $\{ a_1 , a_2 , \cdots , a_k \} $.
We say that an $A$-partition of $n$ into $k$ parts is optimal if there is no $A$-partition of $n$ into $r$ parts with $r<k$. Prove that any optimal $A$-partition of $n$ contains at most $\sqrt[3]{6n}$ different parts.
21 replies
lyukhson
Jul 9, 2014
Ciobi_
2 hours ago
2021 SMT Guts Round 5 p17-20 - Stanford Math Tournament
parmenides51   5
N Today at 8:00 AM by MATHS_ENTUSIAST
p17. Let the roots of the polynomial $f(x) = 3x^3 + 2x^2 + x + 8 = 0$ be $p, q$, and $r$. What is the sum $\frac{1}{p} +\frac{1}{q} +\frac{1}{r}$ ?


p18. Two students are playing a game. They take a deck of five cards numbered $1$ through $5$, shuffle them, and then place them in a stack facedown, turning over the top card next to the stack. They then take turns either drawing the card at the top of the stack into their hand, showing the drawn card to the other player, or drawing the card that is faceup, replacing it with the card on the top of the pile. This is repeated until all cards are drawn, and the player with the largest sum for their cards wins. What is the probability that the player who goes second wins, assuming optimal play?


p19. Compute the sum of all primes $p$ such that $2^p + p^2$ is also prime.


p20. In how many ways can one color the $8$ vertices of an octagon each red, black, and white, such that no two adjacent sides are the same color?


PS. You should use hide for answers. Collected here.
5 replies
parmenides51
Feb 11, 2022
MATHS_ENTUSIAST
Today at 8:00 AM
k Interesting functional equation
IvanRogers1   9
N Today at 5:51 AM by jasperE3
Find all functions $f:  \mathbb{R} \to  \mathbb{R}$ such that $f(x + y) + f(xy) + 1 = f(x) + f(y) + f(xy + 1) \forall x ,y \in \mathbb R$.
9 replies
IvanRogers1
Yesterday at 3:19 PM
jasperE3
Today at 5:51 AM
Compilation of Seq and Series Problems
Saucepan_man02   0
Today at 1:16 AM
Could anyone post some problems/resources on Seq and Series (based on AIME/ARML level)?
0 replies
Saucepan_man02
Today at 1:16 AM
0 replies
2019 SMT Team Round - Stanford Math Tournament
parmenides51   19
N Yesterday at 5:21 PM by SomeonecoolLovesMaths
p1. Given $x + y = 7$, find the value of x that minimizes $4x^2 + 12xy + 9y^2$.


p2. There are real numbers $b$ and $c$ such that the only $x$-intercept of $8y = x^2 + bx + c$ equals its $y$-intercept. Compute $b + c$.



p3. Consider the set of $5$ digit numbers $ABCDE$ (with $A \ne 0$) such that $A+B = C$, $B+C = D$, and $C + D = E$. What’s the size of this set?


p4. Let $D$ be the midpoint of $BC$ in $\vartriangle ABC$. A line perpendicular to D intersects $AB$ at $E$. If the area of $\vartriangle ABC$ is four times that of the area of $\vartriangle BDE$, what is $\angle ACB$ in degrees?


p5. Define the sequence $c_0, c_1, ...$ with $c_0 = 2$ and $c_k = 8c_{k-1} + 5$ for $k > 0$. Find $\lim_{k \to \infty} \frac{c_k}{8^k}$.


p6. Find the maximum possible value of $|\sqrt{n^2 + 4n + 5} - \sqrt{n^2 + 2n + 5}|$.


p7. Let $f(x) = \sin^8 (x) + \cos^8(x) + \frac38 \sin^4 (2x)$. Let $f^{(n)}$ (x) be the $n$th derivative of $f$. What is the largest integer $a$ such that $2^a$ divides $f^{(2020)}(15^o)$?


p8. Let $R^n$ be the set of vectors $(x_1, x_2, ..., x_n)$ where $x_1, x_2,..., x_n$ are all real numbers. Let $||(x_1, . . . , x_n)||$ denote $\sqrt{x^2_1 +... + x^2_n}$. Let $S$ be the set in $R^9$ given by $$S = \{(x, y, z) : x, y, z \in R^3 , 1 = ||x|| = ||y - x|| = ||z -y||\}.$$If a point $(x, y, z)$ is uniformly at random from $S$, what is $E[||z||^2]$?


p9. Let $f(x)$ be the unique integer between $0$ and $x - 1$, inclusive, that is equivalent modulo $x$ to $\left( \sum^2_{i=0} {{x-1} \choose i} ((x - 1 - i)! + i!) \right)$. Let $S$ be the set of primes between $3$ and $30$, inclusive. Find $\sum_{x\in S}^{f(x)}$.


p10. In the Cartesian plane, consider a box with vertices $(0, 0)$,$\left( \frac{22}{7}, 0\right)$,$(0, 24)$,$\left( \frac{22}{7}, 4\right)$. We pick an integer $a$ between $1$ and $24$, inclusive, uniformly at random. We shoot a puck from $(0, 0)$ in the direction of $\left( \frac{22}{7}, a\right)$ and the puck bounces perfectly around the box (angle in equals angle out, no friction) until it hits one of the four vertices of the box. What is the expected number of times it will hit an edge or vertex of the box, including both when it starts at $(0, 0)$ and when it ends at some vertex of the box?


p11. Sarah is buying school supplies and she has $\$2019$. She can only buy full packs of each of the following items. A pack of pens is $\$4$, a pack of pencils is $\$3$, and any type of notebook or stapler is $\$1$. Sarah buys at least $1$ pack of pencils. She will either buy $1$ stapler or no stapler. She will buy at most $3$ college-ruled notebooks and at most $2$ graph paper notebooks. How many ways can she buy school supplies?


p12. Let $O$ be the center of the circumcircle of right triangle $ABC$ with $\angle ACB = 90^o$. Let $M$ be the midpoint of minor arc $AC$ and let $N$ be a point on line $BC$ such that $MN \perp BC$. Let $P$ be the intersection of line $AN$ and the Circle $O$ and let $Q$ be the intersection of line $BP$ and $MN$. If $QN = 2$ and $BN = 8$, compute the radius of the Circle $O$.


p13. Reduce the following expression to a simplified rational $$\frac{1}{1 - \cos \frac{\pi}{9}}+\frac{1}{1 - \cos \frac{5 \pi}{9}}+\frac{1}{1 - \cos \frac{7 \pi}{9}}$$

p14. Compute the following integral $\int_0^{\infty} \log (1 + e^{-t})dt$.


p15. Define $f(n)$ to be the maximum possible least-common-multiple of any sequence of positive integers which sum to $n$. Find the sum of all possible odd $f(n)$


PS. You should use hide for answers. Collected here.
19 replies
parmenides51
Feb 6, 2022
SomeonecoolLovesMaths
Yesterday at 5:21 PM
CSMC Question
vicrong   1
N May 13, 2025 by Mathzeus1024
Prove that there is exactly one function h with the following properties

- the domain of h is the set of positive integers
- h(n) is a positive integer for every positive integer n, and
- h(h(n)+m) = 1+n+h(m) for all positive integers n and m
1 reply
vicrong
Nov 26, 2017
Mathzeus1024
May 13, 2025
Continued fraction
ReticulatedPython   4
N May 12, 2025 by jasperE3
Find the exact value of the continued fraction $$1^2+\frac{1}{2^2+\frac{1}{3^2+\frac{1}{4^2+\frac{1}{5^2+\cdots}}}}
$$
I know that it is approximately $1.2432$ but I am looking for the exact value. Does anyone know how to solve this problem?
4 replies
ReticulatedPython
May 12, 2025
jasperE3
May 12, 2025
Functional equation
TuZo   2
N May 12, 2025 by jasperE3
My question is, if we can determinate or not, all $f:R\to R$ continuous function with $sin(f(x+y))=sin(f(x)+f(y))$ for all real $x,y$.
Thank you!
2 replies
TuZo
Oct 23, 2018
jasperE3
May 12, 2025
Writing/Evaluating Exponential Functions
Samarthsshah   1
N May 11, 2025 by Mathzeus1024
Rewrite the function and determine if the function represents exponential growth or decay. Identify the percent rate of change.

y=2(9)^-x/2
1 reply
Samarthsshah
Jan 30, 2018
Mathzeus1024
May 11, 2025
2023 Official Mock NAIME #15 f(f(f(x))) = f(f(x))
parmenides51   3
N May 9, 2025 by jasperE3
How many non-bijective functions $f$ exist that satisfy $f(f(f(x))) = f(f(x))$ for all real $x$ and the domain of f is strictly within the set of $\{1,2,3,5,6,7,9\}$, the range being $\{1,2,4,6,7,8,9\}$?

Even though this is an AIME problem, a proof is mandatory for full credit. Constants must be ignored as we dont want an infinite number of solutions.
3 replies
parmenides51
Dec 4, 2023
jasperE3
May 9, 2025
one nice!
MihaiT   3
N May 8, 2025 by Pin123
Find positiv integer numbers $(a,b) $ s.t. $\frac{a}{b-2}  $ and $\frac{3b-6}{a-3}$ be positiv integer numbers.
3 replies
MihaiT
Jan 14, 2025
Pin123
May 8, 2025
Reflected point lies on radical axis
Mahdi_Mashayekhi   3
N Apr 19, 2025 by Mahdi_Mashayekhi
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.
3 replies
Mahdi_Mashayekhi
Apr 19, 2025
Mahdi_Mashayekhi
Apr 19, 2025
Reflected point lies on radical axis
G H J
Source: Iran 2025 second round P4
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Mahdi_Mashayekhi
695 posts
#1
Y by
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.
This post has been edited 1 time. Last edited by Mahdi_Mashayekhi, Apr 19, 2025, 11:25 AM
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ItzsleepyXD
149 posts
#2
Y by
Angle chasing :
$\angle BPX = \angle BPA + \angle APX
= 180^{\circ} - \angle BAP - \angle ABO + 90^{\circ} + \angle ACB
= \angle BAC + 90^{\circ}
= 180^{\circ} - \angle O'BC
$
and from $OX//BC$
Let $O'B \cap OX=E , O'B \cap OX = F$
so $\angle BEX + \angle BPX = 180^{\circ}$
thus $E,B,P,X$ concyclic .
in the same way $F,C,P,X$ concyclic.
so $O'$ is on the radical axis of $(BXP),(CXQ)$
implies that $X,Y,O'$ collinear . done $\square$
Z K Y
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gghx
1072 posts
#3 • 1 Y
Y by blackidea
Define $Y'$ as the second intersection of $O'X$ and $(BO'C)$. We claim that $Y=Y'$. Note that $$\angle BY'X=\angle BCO'=\angle BCO=90^\circ-\angle PQO=\angle XPO,$$hence $BPYX'$ is concyclic. Similarly, $CXQY$ is concyclic, and we are done.
Z K Y
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Mahdi_Mashayekhi
695 posts
#4 • 2 Y
Y by Parsia--, sami1618
Note that $\angle XYC = \angle XQC =\angle XQO = 90 - \frac{\angle QXO}{2} = 90 - \angle QPO = 90 - (90 - \angle C + 90 - \angle B) = 90-\angle A$ and $\angle XYB = \angle XPO = 90 - \frac{\angle PXO}{2} = 90 - \angle PQO = 90 - (90 - \angle C + 90 - \angle  B) = 90 - \angle A$ so $X$ lies on angle bisector of $BYC$.
Now note that $\angle BYC = 2(90 - \angle A) = 180 - 2\angle A = 180 - \angle BOC = 180 - \angle BO'C$ so $BYCO'$ is cyclic and since $BO'=CO'$ then $O'$ also lies on angle bisector of $BYC$ so $Y,X,O'$ are collinear as wanted.
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