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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)!

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[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
Thailand MO 2025 P3
Kaimiaku   2
N a few seconds ago by lbh_qys
Let $a,b,c,x,y,z$ be positive real numbers such that $ay+bz+cx \le az+bx+cy$. Prove that $$ \frac{xy}{ax+bx+cy}+\frac{yz}{by+cy+az}+\frac{zx}{cz+az+bx} \le \frac{x+y+z}{a+b+c}$$
2 replies
2 viewing
Kaimiaku
an hour ago
lbh_qys
a few seconds ago
Burapha integer
EeEeRUT   1
N 16 minutes ago by ItzsleepyXD
Source: TMO 2025 P1
For each positive integer $m$, denote by $d(m)$ the number of positive divisors of $m$. We say that a positive integer $n$ is Burapha integer if it satisfy the following condition
[list]
[*] $d(n)$ is an odd integer.
[*] $d(k) \leqslant d(\ell)$ holds for every positive divisor $k, \ell$ of $n$, such that $k < \ell$
[/list]
Find all Burapha integer.
1 reply
EeEeRUT
34 minutes ago
ItzsleepyXD
16 minutes ago
Algebra inequalities
TUAN2k8   1
N 18 minutes ago by lbh_qys
Source: Own
Is that true?
Let $a_1,a_2,...,a_n$ be real numbers such that $0 \leq a_i \leq 1$ for all $1 \leq i \leq n$.
Prove that: $\sum_{1 \leq i<j \leq n} (a_i-a_j)^2 \leq \frac{n}{2}$.
1 reply
1 viewing
TUAN2k8
an hour ago
lbh_qys
18 minutes ago
Quadrilateral with Congruent Diagonals
v_Enhance   37
N 36 minutes ago by Ilikeminecraft
Source: USA TSTST 2012, Problem 2
Let $ABCD$ be a quadrilateral with $AC = BD$. Diagonals $AC$ and $BD$ meet at $P$. Let $\omega_1$ and $O_1$ denote the circumcircle and the circumcenter of triangle $ABP$. Let $\omega_2$ and $O_2$ denote the circumcircle and circumcenter of triangle $CDP$. Segment $BC$ meets $\omega_1$ and $\omega_2$ again at $S$ and $T$ (other than $B$ and $C$), respectively. Let $M$ and $N$ be the midpoints of minor arcs $\widehat {SP}$ (not including $B$) and $\widehat {TP}$ (not including $C$). Prove that $MN \parallel O_1O_2$.
37 replies
v_Enhance
Jul 19, 2012
Ilikeminecraft
36 minutes ago
No more topics!
Combo problem
soryn   3
N Apr 23, 2025 by soryn
The school A has m1 boys and m2 girls, and ,the school B has n1 boys and n2 girls. Each school is represented by one team formed by p students,boys and girls. If f(k) is the number of cases for which,the twice schools has,togheter k girls, fund f(k) and the valute of k, for which f(k) is maximum.
3 replies
soryn
Apr 22, 2025
soryn
Apr 23, 2025
Combo problem
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soryn
5342 posts
#1
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The school A has m1 boys and m2 girls, and ,the school B has n1 boys and n2 girls. Each school is represented by one team formed by p students,boys and girls. If f(k) is the number of cases for which,the twice schools has,togheter k girls, fund f(k) and the valute of k, for which f(k) is maximum.
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soryn
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How cam fiind k, st f(k) is maximum? Who can help me?
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Anulick
173 posts
#5 • 1 Y
Y by soryn
A method using a bit of probability theory could be as follows.

Let's say $X$ is a random variable reprasenting the number of girls in the teams combined. Clearly, $X = X_A + X_B$.

As $X_A$ is independent of $X_B$, $\operatorname{mode}(X) = \operatorname{mode}(X_A) + \operatorname{mode}(X_B)$.

Now notice, $X_A$ and $X_B$ are hyper-geometric, that is
$
\mathrm{P}(X_A = k) = \frac{\binom{m_2}{k} \binom{m_1}{p-k}}{\binom{m_1 + m_2}{p}}
$
and similar for $X_B$. Consider the ratio
\[\frac{\mathrm{P}(X_A = k+1)}{\mathrm{P}(X_A = k)} = 1 \implies k = \left\lfloor \frac{(m_2+1)(p+1)}{m_1+m_2+2} \right\rfloor\]
where we put the floor to get an integer. Note, $a >k$ has $\frac{\mathrm{P}(X_A = a)}{\mathrm{P}(X_A = a)} < 1$ and otherway round for $a < k$.

We do the same for $X_B$ to get the solution that $f(k)$ is maximised at

\[
\boxed{
k = \left\lfloor \frac{(m_2+1)(p+1)}{m_1+m_2+2} \right\rfloor + \left\lfloor \frac{(n_2+1)(p+1)}{n_1+n_2+2} \right\rfloor
}
\]
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soryn
5342 posts
#6
Y by
Interesting! How obtain,exactly,the formula for k? The exactly formula for f(k)? Thx! I think that k is integer part of sum of twice fractions,not thevsum of two floor parts...
This post has been edited 1 time. Last edited by soryn, Apr 23, 2025, 3:22 AM
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