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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.

Are you interested in working towards MATHCOUNTS and don’t know where to start? We have you covered! If you have taken Prealgebra, then you are ready for MATHCOUNTS/AMC 8 Basics. Already aiming for State or National MATHCOUNTS and harder AMC 8 problems? Then our MATHCOUNTS/AMC 8 Advanced course is for you.

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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
Bogus Proof Marathon
pifinity   7636
N an hour ago by TealFrog84
Hi!
I'd like to introduce the Bogus Proof Marathon.

In this marathon, simply post a bogus proof that is middle-school level and the next person will find the error. You don't have to post the real solution :P

Use classic Marathon format:
[hide=P#]a1b2c3[/hide]
[hide=S#]a1b2c3[/hide]


Example posts:

P(x)
-----
S(x)
P(x+1)
-----
Let's go!! Just don't make it too hard!
7636 replies
pifinity
Mar 12, 2018
TealFrog84
an hour ago
INTERSTING
teomihai   0
3 hours ago
IF $1=3$
$2=3$

$3=4$
$4=5 $
FIND $6=?$
0 replies
teomihai
3 hours ago
0 replies
Function Problem
Geometry285   2
N 4 hours ago by Saucepan_man02
The function $f(x)$ can be defined as a sequence such that $x=n$, and $a_n = | a_{n-1} | + \left \lceil \frac{n!}{n^{100}} \right \rceil$, such that $a_n = n$. The function $g(x)$ is such that $g(x) = x!(x+1)!$. How many numbers within the interval $0<n<101$ for the function $g(f(x))$ are perfect squares?
2 replies
Geometry285
Apr 11, 2021
Saucepan_man02
4 hours ago
Prove the statement
Butterfly   11
N 4 hours ago by solyaris
Given an infinite sequence $\{x_n\} \subseteq  [0,1]$, there exists some constant $C$, for any $r>0$, among the sequence $x_n$ and $x_m$ could be chosen to satisfy $|n-m|\ge r $ and $|x_n-x_m|<\frac{C}{|n-m|}$.
11 replies
Butterfly
May 7, 2025
solyaris
4 hours ago
9 Pythagorean Triples
ZMB038   47
N 6 hours ago by pieMax2713
Please put some of the ones you know, and try not to troll/start flame wars! Thank you :D
47 replies
ZMB038
Monday at 6:04 PM
pieMax2713
6 hours ago
External Direct Sum
We2592   0
Today at 2:45 AM
Q) 1. Let $V$ be external direct sum of vector spaces $U$ and $W$ over a field $\mathbb{F}$.let $\hat{U}={\{(u,0):u\in U\}}$ and $\hat{W}={\{(0,w):w\in W\}}$
show that
i) $\hat{U}$ and $\hat{W}$ is subspaces.
ii)$V=\hat{U}\oplus\hat{W}$

Q)2. Suppose $V=U+W$. Let $\hat{V}$ be the external direct sum of $U$ and $W$. show that $V$ is isomorphic to $\hat{V}$ under the correspondence $v=u+w\leftrightarrow(u,w)$

I face some trouble to solve this problems help me for understanding.
thank you.

0 replies
We2592
Today at 2:45 AM
0 replies
Definite integration
girishpimoli   2
N Yesterday at 11:59 PM by Amkan2022
If $\displaystyle g(t)=\int^{t^{2}}_{2t}\cot^{-1}\bigg|\frac{1+x}{(1+t)^2-x}\bigg|dx.$ Then $\displaystyle \frac{g(5)}{g(3)}$ is
2 replies
girishpimoli
Apr 6, 2025
Amkan2022
Yesterday at 11:59 PM
Putnam 1968 A6
sqrtX   11
N Yesterday at 11:47 PM by ohiorizzler1434
Source: Putnam 1968
Find all polynomials whose coefficients are all $\pm1$ and whose roots are all real.
11 replies
sqrtX
Feb 19, 2022
ohiorizzler1434
Yesterday at 11:47 PM
Affine variety
YamoSky   1
N Yesterday at 9:01 PM by amplreneo
Let $A=\left\{z\in\mathbb{C}|Im(z)\geq0\right\}$. Is it possible to equip $A$ with a finitely generated k-algebra with one generator such that make $A$ be an affine variety?
1 reply
YamoSky
Jan 9, 2020
amplreneo
Yesterday at 9:01 PM
Reducing the exponents for good
RobertRogo   0
Yesterday at 6:38 PM
Source: The national Algebra contest (Romania), 2025, Problem 3/Abstract Algebra (a bit generalized)
Let $A$ be a ring with unity such that for every $x \in A$ there exist $t_x, n_x \in \mathbb{N}^*$ such that $x^{t_x+n_x}=x^{n_x}$. Prove that
a) If $t_x \cdot 1 \in U(A), \forall x \in A$ then $x^{t_x+1}=x, \forall x \in A$
b) If there is an $x \in A$ such that $t_x \cdot 1 \notin U(A)$ then the result from a) may no longer hold.

Authors: Laurențiu Panaitopol, Dorel Miheț, Mihai Opincariu, me, Filip Munteanu
0 replies
RobertRogo
Yesterday at 6:38 PM
0 replies
Differential equations , Matrix theory
c00lb0y   3
N Yesterday at 12:26 PM by loup blanc
Source: RUDN MATH OLYMP 2024 problem 4
Any idea?? Diff equational system combined with Matrix theory.
Consider the equation dX/dt=X^2, where X(t) is an n×n matrix satisfying the condition detX=0. It is known that there are no solutions of this equation defined on a bounded interval, but there exist non-continuable solutions defined on unbounded intervals of the form (t ,+∞) and (−∞,t). Find n.
3 replies
c00lb0y
Apr 17, 2025
loup blanc
Yesterday at 12:26 PM
The matrix in some degree is a scalar
FFA21   4
N Yesterday at 12:06 PM by FFA21
Source: MSU algebra olympiad 2025 P2
$A\in M_{3\times 3}$ invertible, for an infinite number of $k$:
$tr(A^k)=0$
Is it true that $\exists n$ such that $A^n$ is a scalar
4 replies
FFA21
Yesterday at 12:11 AM
FFA21
Yesterday at 12:06 PM
Weird integral
Martin.s   0
Yesterday at 9:33 AM
\[
\int_{-\frac{\pi}{2}}^{\frac{\pi}{2}} 
\frac{1 - e^{-2} \cos\left(2\left(u + \tan u\right)\right)}
{1 - 2e^{-2} \cos\left(2\left(u + \tan u\right)\right) + e^{-4}} 
\, \mathrm{d}u
\]
0 replies
Martin.s
Yesterday at 9:33 AM
0 replies
hard number theory problem
danilorj   4
N Yesterday at 9:01 AM by c00lb0y
Let \( a \) and \( b \) be positive integers. Prove that
\[
a^2 + \left\lceil \frac{4a^2}{b} \right\rceil
\]is not a perfect square.
4 replies
danilorj
May 18, 2025
c00lb0y
Yesterday at 9:01 AM
Counting Problems
mithu542   5
N May 7, 2025 by BS2012
Hello!

Here are some challenging practice counting problems. Enjoy! (You're allowed to use a calculator) hint


1.
Yan rolls 9 standard six-sided dice.
What is the probability that at least one pair of dice has a sum of 8?
Round your answer to 3 decimal places.

2.
Each face of a cube is painted one of 5 colors: red, blue, green, yellow, or white.
What is the probability that no two adjacent faces are painted the same color?
Round your answer to 3 decimal places.

3.
You roll 8 standard six-sided dice in a row.
What is the probability that at least one pair of adjacent dice differ by exactly 2?
Round your answer to 3 decimal places.

4.
A 4×4×4 cube (made of 64 mini-cubes) is randomly painted, each mini-cube colored independently either black or white.
What is the probability that at least one mini-cube adjacent to the center mini-cube is black?
Round your answer to 3 decimal places.

5.
Yan rolls 7 dice, each numbered 11 to 88.
What is the probability that at least two dice show the same number?
Round your answer to 3 decimal places.

6.
Each vertex of a cube is randomly colored either red, blue, or green.
What is the probability that there exists at least one face whose four vertices are all the same color?
Round your answer to 3 decimal places.

7.
You roll 6 standard six-sided dice.
What is the probability that the sum of all six dice is divisible by 4?
Round your answer to 3 decimal places.

8.
Each face of a cube is randomly colored red, blue, green, or yellow.
What is the probability that no two opposite faces are painted the same color?
Round your answer to 3 decimal places.

9.
Yan flips a fair coin 12 times.
What is the probability that there is at least one sequence of 4 consecutive heads?
Round your answer to 3 decimal places.

10.
Each edge of a cube is randomly colored either red, blue, or green.
What is the probability that no face of the cube has all three edges the same color?
Round your answer to 3 decimal places.
5 replies
mithu542
Apr 28, 2025
BS2012
May 7, 2025
Counting Problems
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mithu542
1584 posts
#1 • 2 Y
Y by PikaPika999, Exponent11
Hello!

Here are some challenging practice counting problems. Enjoy! (You're allowed to use a calculator) hint


1.
Yan rolls 9 standard six-sided dice.
What is the probability that at least one pair of dice has a sum of 8?
Round your answer to 3 decimal places.

2.
Each face of a cube is painted one of 5 colors: red, blue, green, yellow, or white.
What is the probability that no two adjacent faces are painted the same color?
Round your answer to 3 decimal places.

3.
You roll 8 standard six-sided dice in a row.
What is the probability that at least one pair of adjacent dice differ by exactly 2?
Round your answer to 3 decimal places.

4.
A 4×4×4 cube (made of 64 mini-cubes) is randomly painted, each mini-cube colored independently either black or white.
What is the probability that at least one mini-cube adjacent to the center mini-cube is black?
Round your answer to 3 decimal places.

5.
Yan rolls 7 dice, each numbered 11 to 88.
What is the probability that at least two dice show the same number?
Round your answer to 3 decimal places.

6.
Each vertex of a cube is randomly colored either red, blue, or green.
What is the probability that there exists at least one face whose four vertices are all the same color?
Round your answer to 3 decimal places.

7.
You roll 6 standard six-sided dice.
What is the probability that the sum of all six dice is divisible by 4?
Round your answer to 3 decimal places.

8.
Each face of a cube is randomly colored red, blue, green, or yellow.
What is the probability that no two opposite faces are painted the same color?
Round your answer to 3 decimal places.

9.
Yan flips a fair coin 12 times.
What is the probability that there is at least one sequence of 4 consecutive heads?
Round your answer to 3 decimal places.

10.
Each edge of a cube is randomly colored either red, blue, or green.
What is the probability that no face of the cube has all three edges the same color?
Round your answer to 3 decimal places.
This post has been edited 3 times. Last edited by mithu542, Apr 29, 2025, 9:30 PM
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Bummer12345
150 posts
#2 • 2 Y
Y by PikaPika999, Exponent11
number 1 has to be inspired by that one target p8 question with 6 dice and sum to 7
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Inaaya
405 posts
#3 • 2 Y
Y by PikaPika999, Exponent11
ill solve some of these when i get some of my math and ai4girls done trust
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Math-lover1
304 posts
#4 • 2 Y
Y by PikaPika999, Exponent11
Problem 10 doesn't make sense since each face has 4 edges adjacent to it, not 3.
However, each vertex has 3 edges adjacent to it. If we're considering vertices...

solution to P10 if considering vertices

I might be a bit too late for this one :P
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Math-lover1
304 posts
#5 • 2 Y
Y by PikaPika999, Exponent11
S9
This post has been edited 1 time. Last edited by Math-lover1, May 6, 2025, 6:39 PM
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BS2012
1049 posts
#6 • 3 Y
Y by PikaPika999, Exponent11, MathPerson12321
Math-lover1 wrote:
Problem 10 doesn't make sense since each face has 4 edges adjacent to it, not 3.
However, each vertex has 3 edges adjacent to it. If we're considering vertices...

solution to P10 if considering vertices

I might be a bit too late for this one :P

This is incorrect. In total, there are $3^{12}$ ways to color the edges, so the denominator of the answer, in lowest terms, should divide $3^{12}$ because the probability is the number of successful outcomes over the number of possible outcomes. We have that $9^8=3^{16}$ does not divide $3^{12}.$

In general, linearity of expectation only works for adding expectations, not multiplying them. For example, it is not generally true that $E(XY)=E(X)E(Y)$ for variables $X$ and $Y$ that are not independent.

I think this problem can be done by casework on the colors of the edges on the sides but that seems kinda messy
This post has been edited 5 times. Last edited by BS2012, May 7, 2025, 12:47 AM
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