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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.

Summer camps are starting next month at the Virtual Campus in math and language arts that are 2 - to 4 - weeks in duration. Spaces are still available - don’t miss your chance to have an enriching summer experience. There are middle and high school competition math camps as well as Math Beasts camps that review key topics coupled with fun explorations covering areas such as graph theory (Math Beasts Camp 6), cryptography (Math Beasts Camp 7-8), and topology (Math Beasts Camp 8-9)!

Be sure to mark your calendars for the following upcoming events:
[list][*]May 9th, 4:30pm PT/7:30pm ET, Casework 2: Overwhelming Evidence — A Text Adventure, a game where participants will work together to navigate the map, solve puzzles, and win! All are welcome.
[*]May 19th, 4:30pm PT/7:30pm ET, What's Next After Beast Academy?, designed for students finishing Beast Academy and ready for Prealgebra 1.
[*]May 20th, 4:00pm PT/7:00pm ET, Mathcamp 2025 Qualifying Quiz Part 1 Math Jam, Problems 1 to 4, join the Canada/USA Mathcamp staff for this exciting Math Jam, where they discuss solutions to Problems 1 to 4 of the 2025 Mathcamp Qualifying Quiz!
[*]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
UC Berkeley Integration Bee 2025 Bracket Rounds
Silver08   7
N 30 minutes ago by Aiden-1089
Regular Round

Quarterfinals

Semifinals

3rd Place Match

Finals
7 replies
2 viewing
Silver08
3 hours ago
Aiden-1089
30 minutes ago
f(x+1)-f(x)=f'(x+1/2) implies f(x)=ax^2 +bx+c?
tom-nowy   0
3 hours ago
Source: https://artofproblemsolving.com/community/c4t157249f4h1288200
Is this true?

$f: \mathbb{R} \to \mathbb{R}$ is differentiable and for all $x \in \mathbb{R}, \; f(x+1)-f(x)=f'\left(x+\frac{1}{2}\right)$
$\Longrightarrow f(x)=ax^2 +bx+c$.
0 replies
tom-nowy
3 hours ago
0 replies
Integration Bee Kaizo
Calcul8er   57
N 4 hours ago by Silver08
Hey integration fans. I decided to collate some of my favourite and most evil integrals I've written into one big integration bee problem set. I've been entering integration bees since 2017 and I've been really getting hands on with the writing side of things over the last couple of years. I hope you'll enjoy!
57 replies
Calcul8er
Mar 2, 2025
Silver08
4 hours ago
Problem 2, Grade 12th RMO Shortlist - Year 2002
sticknycu   3
N 6 hours ago by RobertRogo
Let $A \in M_2(C), A \neq O_2, A \neq I_2, n \in \mathbb{N}^*$ and $S_n = \{ X \in M_2(C) | X^n = A \}$.
Show:
a) $S_n$ with multiplication of matrixes operation is making an isomorphic-group structure with $U_n$.
b) $A^2 = A$.

Marian Andronache
3 replies
sticknycu
Jan 3, 2020
RobertRogo
6 hours ago
Circle and square
Marrelia   1
N Apr 10, 2025 by sunken rock
Given a circle with center $O$, and square $ABCD$. Point $A$ and $B$ are on the circle, and $CD$ is tangent to the circle at point $E$. Let $M$ represent the midpoint of $AD$ and $F$ represent the intersection between $AD$ and circle. Prove that $MF = FD$.
1 reply
Marrelia
Apr 10, 2025
sunken rock
Apr 10, 2025
Two geometry and algebra problems
Kempu33334   5
N Apr 9, 2025 by joeym2011
Here are two problems I made...

1) Let there be a triangle $ABC$ such that $AB = 5$, $AC = 6$, and $BC = 7$. Then, let the circumcenter of $\triangle ABC$ be $O$. Furthermore, let the reflections of $A$, $B$, and $C$ across $O$, be $A'$, $B'$, and $C'$ respectively. Find $[AB'CA'BC']$.

2) If \[\underbrace{\dfrac{v_2(4!)}{v_2(8!)}\cdot\dfrac{v_2(16!)}{v_2(32!)}\dots}_\text{50 terms} = \dfrac{1}{2^n}\]find $\left\lceil \dfrac{n}{3} \right\rceil$.

@2below Yes, that’s the intended product.
5 replies
Kempu33334
Apr 8, 2025
joeym2011
Apr 9, 2025
Simple Combinatorics on seating in a circle
duttaditya18   6
N Mar 6, 2025 by S_14159
Five persons wearing badges with numbers $1, 2, 3, 4, 5$ are seated on $5$ chairs around a circular table. In how many ways can they be seated so that no two persons whose badges have consecutive numbers are seated next to each other? (Two arrangements obtained by rotation around the table are considered different)
6 replies
duttaditya18
Aug 11, 2019
S_14159
Mar 6, 2025
Ellipse not parallel to x- or y-axis
chess64   9
N Mar 4, 2025 by daijobu
An ellipse has foci at $(9,20)$ and $(49,55)$ in the $xy$-plane and is tangent to the $x$-axis. What is the length of its major axis?
9 replies
chess64
Jan 1, 2006
daijobu
Mar 4, 2025
[PMO25 Qualifying I.14] Grid Division
kae_3   0
Feb 23, 2025
How many ways are there to divide a $5\times5$ square into three rectangles, all of whose sides are integers? Assume that two configurations which are obtained by either a rotation and/or a reflection are considered the same.

$\text{(a) }10\qquad\text{(b) }12\qquad\text{(c) }14\qquad\text{(d) }16$

Answer Confirmation
0 replies
kae_3
Feb 23, 2025
0 replies
Reflection Geometry
Kempu33334   1
N Feb 17, 2025 by Kempu33334
Let there be a triangle $ABC$, and a point $P$. In addition, let the reflections of $P$ across the sides of $\triangle ABC$ be $X,Y,Z$.

1) Prove that $P$ is in the interior of $\triangle XYZ$ if and only if it is in the interior of $\triangle ABC$.

2.1) Prove that it is impossible for $X$, $Y$, $Z$ to all lie inside (not on) $(ABC)$ or provide a counterexample.

2.2) Prove that it is impossible for $A$, $B$, $C$ to all lie inside (not on) $(XYZ)$ or provide a counterexample.

3) Let all points $P$ such that they lie on a singular circle be the set $\mathcal{P}$. Prove that for every point in $\mathcal{P}$, the locus of the corresponding $X$, $Y$, $Z$ are circles.

4) Let all points $P$ such that they lie on a singular line be the set $\mathcal{L}$. Prove that for every point in $\mathcal{L}$, the locus of the corresponding $X$, $Y$, $Z$ is a line.

5.1) Let all points $P$ such that they lie on a singular ellipse be the set $\mathcal{E}$. Prove that for every point in $\mathcal{E}$, the locus of the corresponding $X$, $Y$, $Z$ is an ellipse.

5.2) Let all points $P$ such that they lie on a singular ellipse be the set $\mathcal{E}$. Prove that for every point in $\mathcal{E}$, the locus of the corresponding $X$, $Y$, $Z$ is an ellipse congruent to $\mathcal{E}$.

Note, 3) and 4) are corollaries of 5), so proving that suffices.

6) Generalize.
1 reply
Kempu33334
Feb 16, 2025
Kempu33334
Feb 17, 2025
Geometry
hn111009   0
Feb 17, 2025
Let triangle $ABC.$ $K$ and $H$ be the reflection of $B$ through $CA$ and $C$ through $BA$. Let $E$ be the center of Euler circle of triangle $ABC.$ Prove that $AE$ passed through the center of $\odot(AKH).$
0 replies
hn111009
Feb 17, 2025
0 replies
intermediate algebra
ANGEW   2
N Jan 28, 2025 by ANGEW
Find all pairs of real numbers (a, b) such that (x - a)^2 + (2x - b)^2 = (x - 3)^2 + (2x)^2 for all x.
The answer is (a,b)=(3,0) and (-9/5, 12/5)
By graphing y=2x and plotting these two points, we can see that the two points are reflections upon the line y=2x. Why?
2 replies
ANGEW
Jan 22, 2025
ANGEW
Jan 28, 2025
Questions about angle notation
tsun26   2
N Jan 13, 2025 by AbhayAttarde01
What's the difference between writing $\angle BAL \cong \angle DAC$ and $\angle BAL = \angle DAC$?
2 replies
tsun26
Jan 13, 2025
AbhayAttarde01
Jan 13, 2025
Properties of Reflections
Dachel2018   3
N Dec 28, 2024 by Mathzeus1024
In Cartesian coordinates, prove that it is impossible to map the point $(0,0)$ to the point $(0,1)$ only by using reflections through lines passing through at least 2 lattice points. Clarifications
3 replies
Dachel2018
Dec 27, 2024
Mathzeus1024
Dec 28, 2024
Dimension of a Linear Space
EthanWYX2009   1
N Apr 19, 2025 by loup blanc
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 $\underline{\quad\quad}.$
1 reply
EthanWYX2009
Apr 19, 2025
loup blanc
Apr 19, 2025
Dimension of a Linear Space
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G H BBookmark kLocked kLocked NReply
Source: 2024 May taca-10
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EthanWYX2009
862 posts
#1 • 1 Y
Y by Euler_Gauss
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 $\underline{\quad\quad}.$
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loup blanc
3595 posts
#2
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$P_u$ is the orthogonal symmetry with respect to $u^{\perp}$.
Let $E=span(v,w),F=E^{\perp}$; then $Q_{| F}=id$ and $Q_{|E}$ is a rotation with angle $\theta\notin \pi\mathbb{Z}$.
Then $Q$ is diagonalizable over $\mathbb{C}$, with the eigenvalues $1$ ($8$ times) and $e^{\pm i\theta}$.
Then the commutant of $Q$ has dimension $8^2+2=66$.
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