Stay ahead of learning milestones! Enroll in a class over the summer!

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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.

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0 replies
jlacosta
Apr 2, 2025
0 replies
Beautiful problem
luutrongphuc   16
N a few seconds ago by Acorn-SJ
Let triangle $ABC$ be circumscribed about circle $(I)$, and let $H$ be the orthocenter of $\triangle ABC$. The circle $(I)$ touches line $BC$ at $D$. The tangent to the circle $(BHC)$ at $H$ meets $BC$ at $S$. Let $J$ be the midpoint of $HI$, and let the line $DJ$ meet $(I)$ again at $X$. The tangent to $(I)$ parallel to $BC$ meets the line $AX$ at $T$. Prove that $ST$ is tangent to $(I)$.
16 replies
luutrongphuc
Apr 4, 2025
Acorn-SJ
a few seconds ago
Stereotypical Diophantine Equation
Mathdreams   2
N 2 minutes ago by grupyorum
Source: 2025 Nepal Mock TST Day 2 Problem 1
Find all solutions in the nonnegative integers to $2^a3^b5^c7^d - 1 = 11^e$.

(Shining Sun, USA)
2 replies
Mathdreams
27 minutes ago
grupyorum
2 minutes ago
A very nice inequality
KhuongTrang   1
N 3 minutes ago by Mathdreams
Source: own
Problem. Let $a,b,c\in \mathbb{R}:\ a+b+c=3.$ Prove that $$\color{black}{\sqrt{5a^{2}-ab+5b^{2}}+\sqrt{5b^{2}-bc+5c^{2}}+\sqrt{5c^{2}-ca+5a^{2}}\le 2(a^2+b^2+c^2)+ab+bc+ca.}$$When does equality hold?
1 reply
1 viewing
KhuongTrang
24 minutes ago
Mathdreams
3 minutes ago
Common tangent to diameter circles
Stuttgarden   1
N 13 minutes ago by jrpartty
Source: Spain MO 2025 P2
The cyclic quadrilateral $ABCD$, inscribed in the circle $\Gamma$, satisfies $AB=BC$ and $CD=DA$, and $E$ is the intersection point of the diagonals $AC$ and $BD$. The circle with center $A$ and radius $AE$ intersects $\Gamma$ in two points $F$ and $G$. Prove that the line $FG$ is tangent to the circles with diameters $BE$ and $DE$.
1 reply
1 viewing
Stuttgarden
Mar 31, 2025
jrpartty
13 minutes ago
No more topics!
Hard number theory
Hip1zzzil   13
N Mar 30, 2025 by Hip1zzzil
Source: FKMO 2025 P6
Positive integers $a, b$ satisfy both of the following conditions.
For a positive integer $m$, if $m^2 \mid ab$, then $m = 1$.
There exist integers $x, y, z, w$ that satisfies the equation $ax^2 + by^2 = z^2 + w^2$ and $z^2 + w^2 > 0$.
Prove that there exist integers $x, y, z, w$ that satisfies the equation $ax^2 + by^2 + n = z^2 + w^2$, for each integer $n$.
13 replies
Hip1zzzil
Mar 30, 2025
Hip1zzzil
Mar 30, 2025
Hard number theory
G H J
G H BBookmark kLocked kLocked NReply
Source: FKMO 2025 P6
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Hip1zzzil
14 posts
#1
Y by
Positive integers $a, b$ satisfy both of the following conditions.
For a positive integer $m$, if $m^2 \mid ab$, then $m = 1$.
There exist integers $x, y, z, w$ that satisfies the equation $ax^2 + by^2 = z^2 + w^2$ and $z^2 + w^2 > 0$.
Prove that there exist integers $x, y, z, w$ that satisfies the equation $ax^2 + by^2 + n = z^2 + w^2$, for each integer $n$.
This post has been edited 4 times. Last edited by Hip1zzzil, Mar 30, 2025, 1:07 PM
Reason: Better
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Acorn-SJ
60 posts
#2 • 1 Y
Y by seoneo
Please copy the original wording, it’s mildly annoying to watch some phrases missing.
In particular, $m$ is supposed to be a positive integer.

EDIT: $n$ is an integer not a positive integer
This post has been edited 1 time. Last edited by Acorn-SJ, Mar 30, 2025, 11:25 AM
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pokmui9909
185 posts
#3 • 1 Y
Y by seoneo
This would be better:
Quote:
Positive integers $a, b$ satisfy both of the following conditions.
  • For a positive integer $m$, if $m^2 \mid ab$, then $m = 1$.
  • There exist integers $x, y, z, w$ that satisfies the equation $ax^2 + by^2 = z^2 + w^2$ and $z^2 + w^2 > 0$.
Prove that there exist integers $x, y, z, w$ that satisfies the equation $ax^2 + by^2 + n = z^2 + w^2$, for each integer $n$.
This post has been edited 2 times. Last edited by pokmui9909, Mar 30, 2025, 7:10 AM
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whwlqkd
80 posts
#4
Y by
Anyone solved it?
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Acorn-SJ
60 posts
#5
Y by
a failed attempt
This post has been edited 1 time. Last edited by Acorn-SJ, Mar 30, 2025, 6:20 AM
Reason: Typo
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Lufin
9 posts
#6 • 2 Y
Y by Acorn-SJ, jjkim0336
In the original paper, n is an integer, and does not have to be a positive integer.
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seoneo
339 posts
#7
Y by
(Sketch)

From the condition, $a$ and $b$ are square free and coprime.
By the infinite descent method, we have relatively prime $a \alpha, b \beta, \zeta, \omega$ so that $a \alpha^2 +b \beta^2 = \zeta^2 + \omega^2$.

Now consider the expression
\[ (\zeta t + p)^2 + (\omega t +q)^2 -a(\alpha t +r)^2 -b(\beta t +s)^2 = 2(\zeta p + \omega q -a\alpha r -b \beta s)t + p^2 +q^2 -ar^2 -bs^2 \]From Bezout, we have $p,q,r,s$ such that
\[ \zeta p + \omega q -a\alpha r -b \beta s =1 \]so we have all ever, or all odd $n$.

By changing parity of $p^2 +q^2 -ar^2 -bs^2 $, we have all integers.

PS. I thought switching the parity would be easy, but it's actually more subtle than I first thought.
This post has been edited 1 time. Last edited by seoneo, Mar 30, 2025, 8:17 AM
Reason: To add PS
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seoneo
339 posts
#8
Y by
This FKMO problem was not accurately translated at first. If the original wording is not preserved in the current post, it may be better to create a properly translated version and redirect others to that post.

P.S. It looks like it's been fixed now.
This post has been edited 1 time. Last edited by seoneo, Mar 31, 2025, 6:31 AM
Reason: To fix grammer.
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segment
24 posts
#10 • 1 Y
Y by MihaiT
seoneo wrote:
(Sketch)

From the condition, $a$ and $b$ are square free and coprime.
By the infinite descent method, we have relatively prime $a \alpha, b \beta, \zeta, \omega$ so that $a \alpha^2 +b \beta^2 = \zeta^2 + \omega^2$.

Now consider the expression
\[ (\zeta t + p)^2 + (\omega t +q)^2 -a(\alpha t +r)^2 -b(\beta t +s)^2 = 2(\zeta p + \omega q -a\alpha r -b \beta s)t + p^2 +q^2 -ar^2 -bs^2 \]From Bezout, we have $p,q,r,s$ such that
\[ \zeta p + \omega q -a\alpha r -b \beta s =1 \]so we have all ever, or all odd $n$.

By changing parity of $p^2 +q^2 -ar^2 -bs^2 $, we have all integers.

Solved same, the motivation was experimenting $a=b=1$, realizing that we can make a linear function of one variable.
I thought \[ \zeta p + \omega q -a\alpha r -b \beta s =1 \]is not enough for the case when $a,b,\alpha,\beta,\gamma,\delta$ are all odd, so for that case I used \[ \zeta p + \omega q -a\alpha r -b \beta s =2 \]and eventually I got all odds, $4k$s, $4k+2$s.
This post has been edited 2 times. Last edited by segment, Mar 30, 2025, 8:06 AM
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MihaiT
748 posts
#11
Y by
very nice!
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GreekIdiot
167 posts
#12
Y by
Acorn-SJ wrote:
Please copy the original wording, it’s mildly annoying to watch some phrases missing.
In particular, $m$ is supposed to be a positive integer.

Doesnt change a lot if $m$ is negative now, does it?
Also in case $ab$ is not squarefree then $m=1$ is a positive integer...
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Acorn-SJ
60 posts
#13
Y by
GreekIdiot wrote:
Acorn-SJ wrote:
Please copy the original wording, it’s mildly annoying to watch some phrases missing.
In particular, $m$ is supposed to be a positive integer.

Doesnt change a lot if $m$ is negative now, does it?
Also in case $ab$ is not squarefree then $m=1$ is a positive integer...


Yes, you could say that, but better preserve the wording for the sake of archiving. Also, a much more important phrase was omitted, so I added that in.
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GreekIdiot
167 posts
#14
Y by
yeah, just saw the edit
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Hip1zzzil
14 posts
#15
Y by
Thank you for all the replies, Sorry for my bad English. I'm working on it though :)
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