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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
rhombus and triangles with same area
Valentin Vornicu   5
N Apr 23, 2005 by perfect_radio
Source: Romanian JBMO TST 2005 - day 2, problem 2
On the sides $AD$ and $BC$ of a rhombus $ABCD$ we consider the points $M$ and $N$ respectively. The line $MC$ intersects the segment $BD$ in the point $T$, and the line $MN$ intersects the segment $BD$ in the point $U$. We denote by $Q$ the intersection between the line $CU$ and the side $AB$ and with $P$ the intersection point between the line $QT$ and the side $CD$.

Prove that the triangles $QCP$ and $MCN$ have the same area.
5 replies
Valentin Vornicu
Apr 2, 2005
perfect_radio
Apr 23, 2005
rhombus and triangles with same area
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Source: Romanian JBMO TST 2005 - day 2, problem 2
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Valentin Vornicu
7301 posts
#1 • 2 Y
Y by Adventure10, Mango247
On the sides $AD$ and $BC$ of a rhombus $ABCD$ we consider the points $M$ and $N$ respectively. The line $MC$ intersects the segment $BD$ in the point $T$, and the line $MN$ intersects the segment $BD$ in the point $U$. We denote by $Q$ the intersection between the line $CU$ and the side $AB$ and with $P$ the intersection point between the line $QT$ and the side $CD$.

Prove that the triangles $QCP$ and $MCN$ have the same area.
This post has been edited 1 time. Last edited by Valentin Vornicu, Apr 2, 2005, 2:09 PM
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nr1337
1213 posts
#2 • 2 Y
Y by Adventure10, Mango247
Valentin Vornicu wrote:
We denote by $Q$ the intersection between the line $AB$ and the side $AB$...

That'd just be AB, right?
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Valentin Vornicu
7301 posts
#3 • 2 Y
Y by Adventure10, Mango247
nr1337 wrote:
Valentin Vornicu wrote:
We denote by $Q$ the intersection between the line $AB$ and the side $AB$...

That'd just be AB, right?
It's $CU$ actually. I've modified the statement. Thanks for noticing.
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probability1.01
2743 posts
#4 • 2 Y
Y by Adventure10, Mango247
Hint:

Click to reveal hidden text
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yetti
2643 posts
#5 • 2 Y
Y by Adventure10, Mango247
Since the triangles $\triangle QCP, \triangle MCN$ have the same altitudes from the vertices $Q, M$ to the sides $CP, CN$, respectively, equal to the distance between the opposite sides of the rhombus $ABCD$, it is necessary to show that $CP = CN$. Using a parallel projection, we can project the rhombus $ABCD$ into a square $A'B'C'D'$, while preserving all area ratios. Hence, it is sufficient to prove the proposition for a square $ABCD$. Let $a = AB$ be the side of the square and $m = \frac{AM}{AD} = \frac{AM}{a}$, $n = \frac{BN}{BC} = \frac{BN}{a}$, $p = \frac{DP}{DC} = \frac{DP}{a}$. We have to show that $n = p$ regardless of $m$.

From similarity of the triangles $\triangle BNU \sim \triangle DMU$, we have $\frac{BU}{DU} = \frac{BN}{DM} = \frac{n}{m - 1}$.

From similarity of the triangles $\triangle BQU \sim \triangle DQU$, we have $\frac{BQ}{DC} = \frac{BU}{DU} = \frac{n}{m - 1}$.

From similarity of the triangles $\triangle BCT \sim \triangle DMT$, we have $\frac{BT}{DT} = \frac{BC}{DM} = \frac{1}{m - 1}$.

From similarity of the triangles $\triangle BTQ \sim \triangle DTP$, we have $\frac{BQ}{DP} = \frac{BT}{DT} = \frac{1}{m - 1}$.

On the other hand, $\frac{BQ}{DP} = \frac{BQ}{DC} \cdot \frac{DC}{DP} = \frac{n}{m - 1} \cdot \frac 1 p$. Thus $\frac{1}{1 - m} = \frac{n}{m - 1} \cdot \frac 1 p$ or $n = p$.
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perfect_radio
2607 posts
#6 • 2 Y
Y by Adventure10, Mango247
you can prove it using just simillarities in the same way, without complicated stuff like parallel projections, which btw i have no idea what they are(no need to assume abcd = square)
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