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Contests & Programs AMC and other contests, summer programs, etc.
AMC and other contests, summer programs, etc.
3 M G
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Arrange marbles
FunGuy1   1
N 6 minutes ago by franklin2013
Source: Own?
Anna has $200$ marbles in $25$ colors such that there are exactly $8$ marbles of each color. She wants to arrange them on $50$ shelves, $4$ marbles on each shelf such that for any $2$ colors there is a shelf that has marbles of those colors.
Can Anna achieve her goal?
1 reply
FunGuy1
16 minutes ago
franklin2013
6 minutes ago
Weird algebra with combinatorial flavour
a_507_bc   5
N 27 minutes ago by BreezeCrowd
Source: Kazakhstan National MO 2023 (10-11).2
Let $n>100$ be an integer. The numbers $1,2 \ldots, 4n$ are split into $n$ groups of $4$. Prove that there are at least $\frac{(n-6)^2}{2}$ quadruples $(a, b, c, d)$ such that they are all in different groups, $a<b<c<d$ and $c-b \leq |ad-bc|\leq d-a$.
5 replies
a_507_bc
Mar 21, 2023
BreezeCrowd
27 minutes ago
Prove the inequality
Butterfly   1
N 29 minutes ago by Royal_mhyasd
Prove that for any positive numbers $x,y$, it holds that
$$x^2+y^2+7\ge 3(x+y)+\frac{9}{xy+2}.$$
1 reply
Butterfly
an hour ago
Royal_mhyasd
29 minutes ago
old and easy imo inequality
Valentin Vornicu   216
N an hour ago by alexanderchew
Source: IMO 2000, Problem 2, IMO Shortlist 2000, A1
Let $ a, b, c$ be positive real numbers so that $ abc = 1$. Prove that
\[ \left( a - 1 + \frac 1b \right) \left( b - 1 + \frac 1c \right) \left( c - 1 + \frac 1a \right) \leq 1.
\]
216 replies
Valentin Vornicu
Oct 24, 2005
alexanderchew
an hour ago
Tangent to incircles.
dendimon18   7
N an hour ago by Gggvds1
Source: ISR 2021 TST1 p.3
Let $ABC$ be an acute triangle with orthocenter $H$. Prove that there is a line $l$ which is parallel to $BC$ and tangent to the incircles of $ABH$ and $ACH$.
7 replies
dendimon18
May 4, 2022
Gggvds1
an hour ago
Problem 3 of RMO 2006 (Regional Mathematical Olympiad-India)
makar   36
N an hour ago by SomeonecoolLovesMaths
Source: Elementry inequality
If $ a,b,c$ are three positive real numbers, prove that $ \frac {a^{2}+1}{b+c}+\frac {b^{2}+1}{c+a}+\frac {c^{2}+1}{a+b}\ge 3$
36 replies
makar
Sep 13, 2009
SomeonecoolLovesMaths
an hour ago
Three numbers cannot be squares simultaneously
WakeUp   40
N an hour ago by Adywastaken
Source: APMO 2011
Let $a,b,c$ be positive integers. Prove that it is impossible to have all of the three numbers $a^2+b+c,b^2+c+a,c^2+a+b$ to be perfect squares.
40 replies
WakeUp
May 18, 2011
Adywastaken
an hour ago
Kaprekar Number
CSJL   5
N an hour ago by Adywastaken
Source: 2025 Taiwan TST Round 1 Independent Study 2-N
Let $k$ be a positive integer. A positive integer $n$ is called a $k$-good number if it satisfies
the following two conditions:

1. $n$ has exactly $2k$ digits in decimal representation (it cannot have leading zeros).

2. If the first $k$ digits and the last $k$ digits of $n$ are considered as two separate $k$-digit
numbers (which may have leading zeros), the square of their sum is equal to $n$.

For example, $2025$ is a $2$-good number because
\[(20 + 25)^2 = 2025.\]Find all $3$-good numbers.
5 replies
CSJL
Mar 6, 2025
Adywastaken
an hour ago
Projective geometry
definite_denny   0
an hour ago
Source: IDK
Let ABC be a triangle and let DEF be the tangency point of incircirle with sides BC,CA,AB. Points P,Q are chosen on sides AB,AC such that PQ is parallel to BC and PQ is tangent to the incircle. Let M denote the midpoint of PQ. Let EF intersect BC at T. Prove that TM is tangent to the incircle
0 replies
definite_denny
an hour ago
0 replies
Problem 7 of RMO 2006 (Regional Mathematical Olympiad-India)
makar   11
N an hour ago by SomeonecoolLovesMaths
Source: Functional Equation
Let $ X$ be the set of all positive integers greater than or equal to $ 8$ and let $ f: X\rightarrow X$ be a function such that $ f(x+y)=f(xy)$ for all $ x\ge 4, y\ge 4 .$ if $ f(8)=9$, determine $ f(9) .$
11 replies
makar
Sep 13, 2009
SomeonecoolLovesMaths
an hour ago
MAN IS KID
DrMath   136
N 2 hours ago by lakshya2009
Source: USAMO 2017 P3, Evan Chen
Let $ABC$ be a scalene triangle with circumcircle $\Omega$ and incenter $I$. Ray $AI$ meets $\overline{BC}$ at $D$ and meets $\Omega$ again at $M$; the circle with diameter $\overline{DM}$ cuts $\Omega$ again at $K$. Lines $MK$ and $BC$ meet at $S$, and $N$ is the midpoint of $\overline{IS}$. The circumcircles of $\triangle KID$ and $\triangle MAN$ intersect at points $L_1$ and $L_2$. Prove that $\Omega$ passes through the midpoint of either $\overline{IL_1}$ or $\overline{IL_2}$.

Proposed by Evan Chen
136 replies
1 viewing
DrMath
Apr 19, 2017
lakshya2009
2 hours ago
System
worthawholebean   10
N Apr 29, 2025 by daijobu
Source: AIME 2008II Problem 14
Let $ a$ and $ b$ be positive real numbers with $ a\ge b$. Let $ \rho$ be the maximum possible value of $ \frac{a}{b}$ for which the system of equations
\[ a^2+y^2=b^2+x^2=(a-x)^2+(b-y)^2\]has a solution in $ (x,y)$ satisfying $ 0\le x<a$ and $ 0\le y<b$. Then $ \rho^2$ can be expressed as a fraction $ \frac{m}{n}$, where $ m$ and $ n$ are relatively prime positive integers. Find $ m+n$.
10 replies
worthawholebean
Apr 3, 2008
daijobu
Apr 29, 2025
System
G H J
Source: AIME 2008II Problem 14
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worthawholebean
3017 posts
#1 • 1 Y
Y by Adventure10
Let $ a$ and $ b$ be positive real numbers with $ a\ge b$. Let $ \rho$ be the maximum possible value of $ \frac{a}{b}$ for which the system of equations
\[ a^2+y^2=b^2+x^2=(a-x)^2+(b-y)^2\]has a solution in $ (x,y)$ satisfying $ 0\le x<a$ and $ 0\le y<b$. Then $ \rho^2$ can be expressed as a fraction $ \frac{m}{n}$, where $ m$ and $ n$ are relatively prime positive integers. Find $ m+n$.
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calc rulz
1126 posts
#2 • 3 Y
Y by Adventure10, Mango247, and 1 other user
Click to reveal hidden text

EDIT: Hmm I think I switched a and b, but the answer is still the same.
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beta
3001 posts
#3 • 2 Y
Y by Adventure10, Mango247
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krustyteklown
641 posts
#4 • 1 Y
Y by Adventure10
These solutions are very nice. I was thinking geometric during the test too, but stuck to normal conic section curves and such.
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K81o7
2417 posts
#5 • 2 Y
Y by Adventure10 and 1 other user
Non-geometric...
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seasonal squirrel
57 posts
#6 • 2 Y
Y by Adventure10, Mango247
i was going through the AOPS wiki and I noticed that K81o7's solution has a small typing error in it. He actually says that the minimum value is 4/3, not the maximum. This doesn't affect the validity of his solution however i do want it to be corrected.
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Mudkipswims42
8867 posts
#7 • 1 Y
Y by Adventure10
calc rulz wrote:
Click to reveal hidden text

EDIT: Hmm I think I switched a and b, but the answer is still the same.


Sorry for the bump, but wouldn't in this solution $f(\theta)$ be maximized at $\theta=0$? This gives a value of $p=\dfrac{2}{\sqrt{3}}$ which matches the other solutions...
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ayushk
362 posts
#8 • 1 Y
Y by Adventure10
He switched $a,b$, which changes the problem a bit.
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yojan_sushi
330 posts
#9 • 3 Y
Y by dknj11902, Adventure10, Mango247
For a non-calculus way to maximize the function in calc rulz's post:
Click to reveal hidden text

This method also applies to maximizing/minimizing functions like $f(x)=\frac{x^2+4x+13}{3x^2+2x+3}$ for $x$ in the reals. See #48 here: http://holbrook.bergen.org/oldcomp/2015/Grade8.pdf
The solution is here: http://holbrook.bergen.org/oldcomp/2015/Grade8sol.pdf
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277546
1607 posts
#10
Y by
Non-Calculus Solution
This post has been edited 2 times. Last edited by 277546, Mar 3, 2020, 7:40 AM
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daijobu
530 posts
#11
Y by
Video Solution
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N Quick Reply
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