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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
Nesbitt inequality
Mathskidd   1
N 13 minutes ago by sqing


$$
$$Would anyone tell me whether the number of ways for proving Nesbitt inequality more than one hundred ?
1 reply
+1 w
Mathskidd
2 hours ago
sqing
13 minutes ago
Indonesia Regional MO 2019 Part A
parmenides51   18
N 17 minutes ago by Mr.Awan
Indonesia Regional MO
Year 2019 Part A

Time: 90 minutes Rules


p1. In the bag there are $7$ red balls and $8$ white balls. Audi took two balls at once from inside the bag. The chance of taking two balls of the same color is ...


p2. Given a regular hexagon with a side length of $1$ unit. The area of the hexagon is ...


p3. It is known that $r, s$ and $1$ are the roots of the cubic equation $x^3 - 2x + c = 0$. The value of $(r-s)^2$ is ...


p4. The number of pairs of natural numbers $(m, n)$ so that $GCD(n,m) = 2$ and $LCM(m,n) = 1000$ is ...


p5. A data with four real numbers $2n-4$, $2n-6$, $n^2-8$, $3n^2-6$ has an average of $0$ and a median of $9/2$. The largest number of such data is ...


p6. Suppose $a, b, c, d$ are integers greater than $2019$ which are four consecutive quarters of an arithmetic row with $a <b <c <d$. If $a$ and $d$ are squares of two consecutive natural numbers, then the smallest value of $c-b$ is ...


p7. Given a triangle $ABC$, with $AB = 6$, $AC = 8$ and $BC = 10$. The points $D$ and $E$ lies on the line segment $BC$. with $BD = 2$ and $CE = 4$. The measure of the angle $\angle DAE$ is ...


p8. Sequqnce of real numbers $a_1,a_2,a_3,...$ meet $\frac{na_1+(n-1)a_2+...+2a_{n-1}+a_n}{n^2}=1$ for each natural number $n$. The value of $a_1a_2a_3...a_{2019}$ is ....


p9. The number of ways to select four numbers from $\{1,2,3, ..., 15\}$ provided that the difference of any two numbers at least $3$ is ...


p10. Pairs of natural numbers $(m , n)$ which satisfies $$m^2n+mn^2 +m^2+2mn = 2018m + 2019n + 2019$$are as many as ...


p11. Given a triangle $ABC$ with $\angle ABC =135^o$ and $BC> AB$. Point $D$ lies on the side $BC$ so that $AB=CD$. Suppose $F$ is a point on the side extension $AB$ so that $DF$ is perpendicular to $AB$. The point $E$ lies on the ray $DF$ such that $DE> DF$ and $\angle ACE = 45^o$. The large angle $\angle AEC$ is ...


p12. The set of $S$ consists of $n$ integers with the following properties: For every three different members of $S$ there are two of them whose sum is a member of $S$. The largest value of $n$ is ....


p13. The minimum value of $\frac{a^2+2b^2+\sqrt2}{\sqrt{ab}}$ with $a, b$ positive reals is ....


p14. The polynomial P satisfies the equation $P (x^2) = x^{2019} (x+ 1) P (x)$ with $P (1/2)= -1$ is ....


p15. Look at a chessboard measuring $19 \times 19$ square units. Two plots are said to be neighbors if they both have one side in common. Initially, there are a total of $k$ coins on the chessboard where each coin is only loaded exactly on one square and each square can contain coins or blanks. At each turn. You must select exactly one plot that holds the minimum number of coins in the number of neighbors of the plot and then you must give exactly one coin to each neighbor of the selected plot. The game ends if you are no longer able to select squares with the intended conditions. The smallest number of $k$ so that the game never ends for any initial square selection is ....
18 replies
parmenides51
Nov 11, 2021
Mr.Awan
17 minutes ago
Algebra Problems
ilikemath247365   10
N 3 hours ago by lgx57
Find all real $(a, b)$ with $a + b = 1$ such that

$(a + \frac{1}{a})^{2} + (b + \frac{1}{b})^{2} = \frac{25}{2}$.
10 replies
ilikemath247365
Apr 14, 2025
lgx57
3 hours ago
Find all triples
pedronis   1
N 4 hours ago by MathRook7817
Find all triples of positive integers $(n, r, s)$ such that $n^2 + n + 1$ divides $n^r + n^s + 1$.
1 reply
pedronis
Today at 12:14 AM
MathRook7817
4 hours ago
Dimension of a Linear Space
EthanWYX2009   0
5 hours ago
Source: 2024 May taca-10
Let \( V \) be a $10$-dimensional inner product space of column vectors, where for \( v = (v_1, v_2, \dots, v_{10})^T \) and \( w = (w_1, w_2, \dots, w_{10})^T \), the inner product of \( v \) and \( w \) is defined as \[\langle v, w \rangle = \sum_{i=1}^{10} v_i w_i.\]For \( u \in V \), define a linear transformation \( P_u \) on \( V \) as follows:
\[ P_u : V \to V, \quad x \mapsto x - \frac{2\langle x, u \rangle u}{\langle u, u \rangle} \]Given \( v, w \in V \) satisfying
\[ 0 < \langle v, w \rangle < \sqrt{\langle v, v \rangle \langle w, w \rangle} \]let \( Q = P_v \circ P_w \). Then the dimension of the linear space formed by all linear transformations \( P : V \to V \) satisfying \( P \circ Q = Q \circ P \) is $\underline{\quad\quad}.$
0 replies
EthanWYX2009
5 hours ago
0 replies
Matrices and Determinants
Saucepan_man02   5
N Today at 1:23 AM by Saucepan_man02
Hello

Can anyone kindly share some problems/handouts on matrices & determinants (problems like Putnam 2004 A3, which are simple to state and doesnt involve heavy theory)?

Thank you..
5 replies
Saucepan_man02
Apr 4, 2025
Saucepan_man02
Today at 1:23 AM
Jordan form and canonical base of a matrix
And1viper   2
N Today at 12:49 AM by rchokler
Find the Jordan form and a canonical basis of the following matrix $A$ over the field $Z_5$:
$$A = \begin{bmatrix}
    2 & 1 & 2 & 0 & 0 \\
    0 & 4 & 0 & 3 & 4 \\
    0 & 0 & 2 & 1 & 2 \\
    0 & 0 & 0 & 4 & 1 \\
    0 & 0 & 0 & 0 & 2
  \end{bmatrix}
$$
2 replies
And1viper
Feb 26, 2023
rchokler
Today at 12:49 AM
Putnam 1960 B1
sqrtX   4
N Yesterday at 11:26 PM by KAME06
Source: Putnam 1960
Find all integer solutions $(m,n)$ to $m^{n}=n^{m}.$
4 replies
sqrtX
Jun 18, 2022
KAME06
Yesterday at 11:26 PM
Putnam 1958 November B1
sqrtX   11
N Yesterday at 11:09 PM by Hello_Kitty
Source: Putnam 1958 November
Given
$$b_n = \sum_{k=0}^{n} \binom{n}{k}^{-1}, \;\; n\geq 1,$$prove that
$$b_n = \frac{n+1}{2n} b_{n-1} +1, \;\; n \geq 2.$$Hence, as a corollary, show
$$ \lim_{n \to \infty} b_n =2.$$
11 replies
sqrtX
Jul 19, 2022
Hello_Kitty
Yesterday at 11:09 PM
2025 OMOUS Problem 6
enter16180   1
N Yesterday at 10:43 PM by Doru2718
Source: Open Mathematical Olympiad for University Students (OMOUS-2025)
Let $A=\left(a_{i j}\right)_{i, j=1}^{n} \in M_{n}(\mathbb{R})$ be a positive semi-definite matrix. Prove that the matrix $B=\left(b_{i j}\right)_{i, j=1}^{n} \text {, where }$ $b_{i j}=\arcsin \left(x^{i+j}\right) \cdot a_{i j}$, is also positive semi-definite for all $x \in(0,1)$.
1 reply
enter16180
Yesterday at 11:52 AM
Doru2718
Yesterday at 10:43 PM
2025 OMOUS Problem 1
enter16180   1
N Yesterday at 7:02 PM by KAME06
Source: Open Mathematical Olympiad for University Students (OMOUS-2025)
Aman and Berdi, two biologists, they invented a new type of bacteria such that they can control the division of bacteria into several parts. They are also participants of $OMOUS-2025$ with the aim to train for the first problem of $OMOUS-2025$. They play the following game.
Initially, they take $1$ bacteria and choose a natural number $n$. On each move, the player chooses any $k$ number from $1$ to $n$. Then the player divides each bacterium into $k$ pants. Once chosen, the number $k$ cannot be chosen twice. If after any player's move the number of bacteria population is divisible by $n$ then that player loses. Determine who has the winning strategy depending on the given number $n$ if it's known that Amman starts first.
1 reply
enter16180
Yesterday at 11:44 AM
KAME06
Yesterday at 7:02 PM
Interesting Limit
Riptide1901   0
Yesterday at 6:18 PM
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 $f(x)=x^x.$
0 replies
Riptide1901
Yesterday at 6:18 PM
0 replies
Sequence of functions
Squeeze   1
N Yesterday at 5:08 PM by Squeeze
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.
1 reply
Squeeze
Yesterday at 3:56 AM
Squeeze
Yesterday at 5:08 PM
Integrate lnx/sqrt{1-x^2}
EthanWYX2009   1
N Yesterday at 3:43 PM by GreenKeeper
Determine the value of
\[I=\int\limits_{0}^{1}\frac{\ln x}{\sqrt{1-x^2}}\mathrm dx.\]
1 reply
EthanWYX2009
Yesterday at 2:38 PM
GreenKeeper
Yesterday at 3:43 PM
k Geometry / Computation
duttaditya18   81
N Aug 12, 2019 by brainiacmaniac31
An ant leaves the anthill for its morning exercise. It walks $4$ feet east and then makes a $160^\circ$ turn to the right and walks $4$ more feet. If the ant continues this patterns until it reaches the anthill again, what is the distance in feet it would have walked?
81 replies
duttaditya18
Aug 11, 2019
brainiacmaniac31
Aug 12, 2019
Geometry / Computation
G H J
G H BBookmark kLocked kLocked NReply
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duttaditya18
59 posts
#1 • 2 Y
Y by Adventure10, Mango247
An ant leaves the anthill for its morning exercise. It walks $4$ feet east and then makes a $160^\circ$ turn to the right and walks $4$ more feet. If the ant continues this patterns until it reaches the anthill again, what is the distance in feet it would have walked?
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Smoothe
176 posts
#2 • 2 Y
Y by Adventure10, Mango247
Ans is 72.
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GeoMetrix
924 posts
#4 • 1 Y
Y by Adventure10
$72$ obviously
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duttaditya18
59 posts
#5 • 1 Y
Y by Adventure10
@amaanmathbuddy_2006 are you sure it is not $76$ because he had moved $4$ at the start, then the ant started moving $20^{\circ}$ every turn.
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Smoothe
176 posts
#6 • 2 Y
Y by Adventure10, Mango247
duttaditya18 wrote:
@amaanmathbuddy_2006 are you sure it is not $76$ because he had moved $4$ at the start, then the ant started moving $20^{\circ}$ every turn.

Sure
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VERYunfortunate
3 posts
#7 • 4 Y
Y by Adventure10, Mango247, Mango247, Mango247
Shouldn't a 160 degree turn be something like $\angle$
This post has been edited 4 times. Last edited by VERYunfortunate, Aug 11, 2019, 10:29 AM
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Akash_Arjun
4 posts
#8 • 2 Y
Y by Adventure10, Mango247
Please show a detailed solution to this problem as I am a beginner.
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Rishi123456
51 posts
#11 • 2 Y
Y by Adventure10, Mango247
Smoothe wrote:
Smoothe wrote:
Ans is 72.

Please check.

Easy question answer will be 72
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zuss77
520 posts
#12 • 1 Y
Y by Adventure10
duttaditya18 wrote:
An ant leaves the anthill for its morning exercise. It walks $4$ feet east and then makes a $160^\circ$ turn to the right and walks $4$ more feet. If the ant continues this patterns until it reaches the anthill again, what is the distance in feet it would have walked?

What ridiculous question is that? Obviously ant never returns to the anthill.
Attachments:
This post has been edited 1 time. Last edited by zuss77, Aug 11, 2019, 1:57 PM
Reason: diagram
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rockin.numbers
59 posts
#13 • 4 Y
Y by Janstew, Quantum_fluctuations, RudraRockstar, Adventure10
I’ve got 36 and it should be completed in 9 rounds. Try to draw out the diagram
This post has been edited 1 time. Last edited by rockin.numbers, Aug 11, 2019, 1:32 PM
Reason: Left out some part.
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Quantum_fluctuations
1282 posts
#14 • 2 Y
Y by Janstew, Adventure10
Yes, it is 36.
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mathsworm
765 posts
#15 • 1 Y
Y by Adventure10
Hey please tell the answer
Some of my friends got 24,36,72
I did 72 by the way
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UjwalKumar
58 posts
#16 • 4 Y
Y by RudraRockstar, Agsh2005, killadg, Adventure10
According to me, the answer is obv 36

The ant follows the diagonals of a nonagon.
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zuss77
520 posts
#17 • 1 Y
Y by Adventure10
Read more carefully.

Actually, I guess it opposite: don't read carefully, if you want to get good score.
This post has been edited 1 time. Last edited by zuss77, Aug 11, 2019, 2:16 PM
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Fredtheidiot
12 posts
#18 • 2 Y
Y by Adventure10, Mango247
emmm,why do I get 156 :roll:
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