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k a April Highlights and 2025 AoPS Online Class Information
jlacosta   0
Apr 2, 2025
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0 replies
jlacosta
Apr 2, 2025
0 replies
Equal Distances in an Isosceles Setting
mojyla222   3
N 7 minutes ago by sami1618
Source: IDMC 2025 P4
Let $ABC$ be an isosceles triangle with $AB=AC$. The circle $\omega_1$, passing through $B$ and $C$, intersects segment $AB$ at $K\neq B$. The circle $\omega_2$ is tangent to $BC$ at $B$ and passes through $K$. Let $M$ and $N$ be the midpoints of segments $AB$ and $AC$, respectively. The line $MN$ intersects $\omega_1$ and $\omega_2$ at points $P$ and $Q$, respectively, where $P$ and $Q$ are the intersections closer to $M$. Prove that $MP=MQ$.

Proposed by Hooman Fattahi
3 replies
mojyla222
Today at 5:05 AM
sami1618
7 minutes ago
standard Q FE
jasperE3   1
N 11 minutes ago by ErTeeEs06
Source: gghx, p19004309
Find all functions $f:\mathbb Q\to\mathbb Q$ such that for any $x,y\in\mathbb Q$:
$$f(xf(x)+f(x+2y))=f(x)^2+f(y)+y.$$
1 reply
jasperE3
3 hours ago
ErTeeEs06
11 minutes ago
Dear Sqing: So Many Inequalities...
hashtagmath   33
N 11 minutes ago by GeoMorocco
I have noticed thousands upon thousands of inequalities that you have posted to HSO and was wondering where you get the inspiration, imagination, and even the validation that such inequalities are true? Also, what do you find particularly appealing and important about specifically inequalities rather than other branches of mathematics? Thank you :)
33 replies
hashtagmath
Oct 30, 2024
GeoMorocco
11 minutes ago
3 knightlike moves is enough
sarjinius   1
N 22 minutes ago by markam
Source: Philippine Mathematical Olympiad 2025 P6
An ant is on the Cartesian plane. In a single move, the ant selects a positive integer $k$, then either travels [list]
[*] $k$ units vertically (up or down) and $2k$ units horizontally (left or right); or
[*] $k$ units horizontally (left or right) and $2k$ units vertically (up or down).
[/list]
Thus, for any $k$, the ant can choose to go to one of eight possible points.
Prove that, for any integers $a$ and $b$, the ant can travel from $(0, 0)$ to $(a, b)$ using at most $3$ moves.
1 reply
sarjinius
Mar 9, 2025
markam
22 minutes ago
Find the answer
JetFire008   1
N 3 hours ago by Filipjack
Source: Putnam and Beyond
Find all pairs of real numbers $(a,b)$ such that $ a\lfloor bn \rfloor = b\lfloor an \rfloor$ for all positive integers $n$.
1 reply
JetFire008
Today at 12:31 PM
Filipjack
3 hours ago
Pyramid packing in sphere
smartvong   2
N 5 hours ago by smartvong
Source: own
Let $A_1$ and $B$ be two points that are diametrically opposite to each other on a unit sphere. $n$ right square pyramids are fitted along the line segment $\overline{A_1B}$, such that the apex and altitude of each pyramid $i$, where $1\le i\le n$, are $A_i$ and $\overline{A_iA_{i+1}}$ respectively, and the points $A_1, A_2, \dots, A_n, A_{n+1}, B$ are collinear.

(a) Find the maximum total volume of $n$ pyramids, with altitudes of equal length, that can be fitted in the sphere, in terms of $n$.

(b) Find the maximum total volume of $n$ pyramids that can be fitted in the sphere, in terms of $n$.

(c) Find the maximum total volume of the pyramids that can be fitted in the sphere as $n$ tends to infinity.

Note: The altitudes of the pyramids are not necessarily equal in length for (b) and (c).
2 replies
smartvong
Apr 13, 2025
smartvong
5 hours ago
Converging product
mathkiddus   5
N Today at 2:40 PM by NamelyOrange
Source: mathkiddus
Evaluate the infinite product, $$\prod_{n=1}^{\infty} \frac{7^n - n}{7^n + n}.$$
5 replies
mathkiddus
Apr 18, 2025
NamelyOrange
Today at 2:40 PM
Interesting Limit
Riptide1901   1
N Today at 1:45 PM by Svyatoslav
Find $\displaystyle\lim_{x\to\infty}\left|f(x)-\Gamma^{-1}(x)\right|$ where $\Gamma^{-1}(x)$ is the inverse gamma function, and $f^{-1}$ is the inverse of $f(x)=x^x.$
1 reply
Riptide1901
Apr 18, 2025
Svyatoslav
Today at 1:45 PM
2022 Putnam B1
giginori   25
N Today at 12:13 PM by cursed_tangent1434
Suppose that $P(x)=a_1x+a_2x^2+\ldots+a_nx^n$ is a polynomial with integer coefficients, with $a_1$ odd. Suppose that $e^{P(x)}=b_0+b_1x+b_2x^2+\ldots$ for all $x.$ Prove that $b_k$ is nonzero for all $k \geq 0.$
25 replies
giginori
Dec 4, 2022
cursed_tangent1434
Today at 12:13 PM
Number of A^2=I3
EthanWYX2009   1
N Today at 11:21 AM by loup blanc
Source: 2025 taca-14
Determine the number of $A\in\mathbb F_5^{3\times 3}$, such that $A^2=I_3.$
1 reply
EthanWYX2009
Today at 7:51 AM
loup blanc
Today at 11:21 AM
Prove this recursion!
Entrepreneur   3
N Today at 11:06 AM by quasar_lord
Source: Amit Agarwal
Let $$I_n=\int z^n e^{\frac 1z}dz.$$Prove that $$\color{blue}{I_n=(n+1)!I_0+e^{\frac 1z}\sum_{n=1}^n n! z^{n+1}.}$$
3 replies
Entrepreneur
Jul 31, 2024
quasar_lord
Today at 11:06 AM
Pove or disprove
Butterfly   1
N Today at 10:05 AM by Filipjack

Denote $y_n=\max(x_n,x_{n+1},x_{n+2})$. Prove or disprove that if $\{y_n\}$ converges then so does $\{x_n\}.$
1 reply
Butterfly
Today at 9:34 AM
Filipjack
Today at 10:05 AM
fractional binomial limit sum
Levieee   3
N Today at 9:44 AM by Levieee
this was given to me by a friend

$\lim_{n \to \infty} \sum_{k=1}^{n}{\frac{1}{\binom{n}{k}}}$

a nice solution using sandwich is
$\frac{1}{n}   + \frac{1}{n} + 1 + \frac{n-3}{\binom{n}{2}} \ge \frac{1}{n} +  \sum_{k=2}^{n-2}{\frac{1}{\binom{n}{k}}}+ \frac{1}{n} + 1 \ge \frac{1}{n} +  + \frac{1}{n} + 1$

therefore $\lim_{n \to \infty} \sum_{k=1}^{n}{\frac{1}{\binom{n}{k}}}$ = $1$

ALSO ANOTHER SOLUTION WHICH I WAS THINKING OF WITHOUT SANDWICH BUT I CANT COMPLETE WAS TO USE THE GAMMA FUNCTION

we know

$B(x, y) = \int_0^1 t^{x - 1} (1 - t)^{y - 1} \, dt$

$B(x, y) = \frac{\Gamma(x)\Gamma(y)}{\Gamma(x + y)}$

and $\Gamma(n) = (n-1)!$ for integers,

$\frac{1}{\binom{n}{k}}$ = $\frac{k! (n-k)!}{n!}$

therefore from the gamma function we get

$ (n+1) \int_{0}^{1}  x^k (1-x)^{n-k} dx$ = $\frac{1}{\binom{n}{k}}$ = $\frac{k! (n-k)!}{n!}$
$\Rightarrow$ $\lim_{n \to \infty} (n+1) \int_{0}^{1} \sum_{k=1}^{n} x^k (1-x)^{n-k} dx$ $=\lim_{n \to \infty} \sum_{k=1}^{n}{\frac{1}{\binom{n}{k}}}$

somehow im supposed to show that

$\lim_{n \to \infty} (n+1) \int_{0}^{1} \sum_{k=1}^{n} x^k (1-x)^{n-k} dx$ $= 1$

all i could observe was if we do L'hopital (which i hate to do as much as you do)

i get $\frac{ \int_{0}^{1} \sum_{k=1}^{n} x^k (1-x)^{n-k} dx}{1/n+1}$

now since $x \in (0,1)$ , as $n \to \infty$ the $(1-x)^{n-k} \to 0$ which gets us the $\frac{0}{0}$ form therefore L'hopital came to my mind , which might be a completely wrong intuition, anyway what should i do to find that limit

:noo: :pilot:
3 replies
Levieee
Yesterday at 9:51 PM
Levieee
Today at 9:44 AM
Integrate exp(x-10cosh(2x))
EthanWYX2009   2
N Today at 8:01 AM by Moubinool
Source: 2024 May taca-14
Determine the value of
\[I=\int\limits_{-\infty}^{\infty}e^{x-10\cosh (2x)}\mathrm dx.\]
2 replies
EthanWYX2009
Apr 18, 2025
Moubinool
Today at 8:01 AM
AE is equal to inradius of triangle ABC
shohvanilu   8
N Sep 3, 2023 by IAmTheHazard
Source: All-Russian Olympiad 2012 Grade 10 Day 1, UZMO 2012
The inscribed circle $\omega$ of the non-isosceles acute-angled triangle $ABC$ touches the side $BC$ at the point $D$. Suppose that $I$ and $O$ are the centres of inscribed circle and circumcircle of triangle $ABC$ respectively. The circumcircle of triangle $ADI$ intersects $AO$ at the points $A$ and $E$. Prove that $AE$ is equal to the radius $r$ of $\omega$.
8 replies
shohvanilu
May 29, 2012
IAmTheHazard
Sep 3, 2023
AE is equal to inradius of triangle ABC
G H J
Source: All-Russian Olympiad 2012 Grade 10 Day 1, UZMO 2012
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shohvanilu
274 posts
#1 • 1 Y
Y by Adventure10
The inscribed circle $\omega$ of the non-isosceles acute-angled triangle $ABC$ touches the side $BC$ at the point $D$. Suppose that $I$ and $O$ are the centres of inscribed circle and circumcircle of triangle $ABC$ respectively. The circumcircle of triangle $ADI$ intersects $AO$ at the points $A$ and $E$. Prove that $AE$ is equal to the radius $r$ of $\omega$.
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RSM
736 posts
#2 • 3 Y
Y by MahsaMorady, Adventure10, Mango247
$ AO $ is the reflection of $ ID $ on the perpendicular bisector of $ AI $, since $ ID $ and $ AO $ are anti-parallel wrt $ \angle BAC $. Now since $ AEDI $ is cyclic. So $ AEDI $ is an isosceles trapezium. So $ AE=r $.
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hatchguy
555 posts
#3 • 2 Y
Y by Adventure10, Fardad
We approach indirectly.

Take a point $E'$ in $AO$ such that $A$ lies between $E'$ and $O$ and $AE' = r$.

If we show that $AI \parallel E'D$ we will be done since this implies $AE'DI$ being cyclic and therefore $E' = E$.

Denote by $L$ the intersection of $AO$ and $ID$. Let $N \equiv AI \cap BC$. It is enough to show that $LA = LI$.

The last is equivalent to $\angle IAO = \angle LIA = \angle DIN$.

So we only need to show that $\angle IAO + \angle INB = 90$ which is just simple angle chase.

EDIT: Weird, RSM's post didnt appear when I wrote my solution, and it was posted some hours ago...
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WakeUp
1347 posts
#4 • 2 Y
Y by Adventure10, Mango247
hatchguy wrote:
EDIT: Weird, RSM's post didnt appear when I wrote my solution, and it was posted some hours ago...
What happened was that I posted this problem (it's a problem from the ARO 2012), but later a fellow Geo moderator brought to my attention that it had already been posted recently. So I was about to delete my thread and replace it on the resources page with this one, however you posted your solution before I could :) So I moved your post here and deleted the old thread.
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anantmudgal09
1979 posts
#5 • 2 Y
Y by Adventure10, Mango247
Let $K$ be the intersection of the $A$ altitude with $\odot(ADI)$. Since $AK, AO$ are symmetric in $AI$, we get $\overarc{IE}=\overarc{IK}$. From $AK \parallel DI$ we get $\overarc{AI}=\overarc{DK} \Longrightarrow \overarc{AE}=\overarc{ID}$ so $AE=r$ as required.
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jayme
9777 posts
#6 • 2 Y
Y by Adventure10, Mango247
Dear Mathlinkers,
also at

http://www.artofproblemsolving.com/Forum/viewtopic.php?f=46&t=481639

Sincerely
Jean-Louis
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Synthetic_Potato
114 posts
#8 • 2 Y
Y by Adventure10, Mango247
WoLoG, assume that $AB>AC$. Let $ID\cap AO = P$. See that $\angle PIA = \angle IAC + \angle ACD + \angle CDI - 180^\circ = \angle C + \frac{\angle A}2 - 90^\circ = \frac{\angle A}2 - (90^\circ - \angle C) = \angle IAB - \angle OAB = \angle PAI$. Hence, $\angle PAI = \angle PIA$. Now the conclusion is obvious.
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mathetillica
333 posts
#10 • 1 Y
Y by Adventure10
Since $AEDI$ is cyclic so $\angle AID=360-\angle B-\frac{\angle A}{2}-90=270-\angle B- \frac{\angle A}{2}=180-\angle AED$ which implies $\angle AED=\frac{\angle A}{2}-(90-\angle B)=\angle OAI$.Thus $AI||ED$ which gives $AE=ID=r$ as $AEDI$ now becomes isosceles trapezium.
This post has been edited 6 times. Last edited by mathetillica, Jan 7, 2019, 7:51 AM
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IAmTheHazard
5001 posts
#11 • 1 Y
Y by centslordm
...

Let $H$ be the orthocenter. Then since $H$ and $O$ are isogonal conjugates,
$$\measuredangle AID=\measuredangle IAH=\measuredangle OAI=\measuredangle EAI,$$hence $AIDE$ is an isosceles trapezoid, so $AE=ID$. $\blacksquare$
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