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9 Can I make MOP
Bigtree   27
N 2 hours ago by ethan2011
My dream is to be on IMO team ik thats not going to happen b/c the kids that make it are like 6th mop quals :play_ball:. I somehow got a $208.5$ index this yr (118.5 on amc10+ 9 on AIME) i’m in 7th rn btw first year comp math also. I will grind so hard until like 30 hrs/week. I’m ok at proofs. made mc nats
27 replies
Bigtree
Mar 9, 2025
ethan2011
2 hours ago
Erecting Rectangles
franchester   102
N 3 hours ago by endless_abyss
Source: 2021 USAMO Problem 1/2021 USAJMO Problem 2
Rectangles $BCC_1B_2,$ $CAA_1C_2,$ and $ABB_1A_2$ are erected outside an acute triangle $ABC.$ Suppose that \[\angle BC_1C+\angle CA_1A+\angle AB_1B=180^{\circ}.\]Prove that lines $B_1C_2,$ $C_1A_2,$ and $A_1B_2$ are concurrent.
102 replies
franchester
Apr 15, 2021
endless_abyss
3 hours ago
What should I do
Jaxman8   1
N 5 hours ago by neeyakkid23
I recently mocked 2 AMC 10’s, and 2 AIME’s. My scores for the AMC 10 were both 123 and my AIME scores were 8 and 9 for 2010 I and II. What should I study for 2025-2026 AMCs? Goal is JMO.
1 reply
Jaxman8
Today at 5:08 AM
neeyakkid23
5 hours ago
Practice AMC 10 Final Fives
freddyfazbear   1
N Today at 5:21 AM by WannabeUSAMOkid
So someone pointed out to me that the last five problems on my previous practice AMC 10 test were rather low quality. Here are some problems that are (hopefully) better.

21.
A partition of a positive integer n is writing n as the sum of positive integer(s), where order does not matter. Find the number of partitions of 6.
A - 10, B - 11, C - 12, D - 13, E - 14

22.
Let n be the smallest positive integer that satisfies the following conditions:
- n is even
- The last digit of n is not 2 or 8
- n^2 + 1 is composite
Find the sum of the digits of n.
A - 3, B - 5, C - 8, D - 9, E - 10

23.
Find the sum of the coordinates of the reflection of the point (6, 9) over the line x + 2y + 3 = 0.
A - (-17.7), B - (-17.6), C - (-17.5), D - (-17.4), E - (-17.3)

24.
Find the number of ordered pairs of integers (a, b), where both a and b have absolute value less than 69, such that a^2 + 42b^2 = 13ab.
A - 21, B - 40, C - 41, D - 42, E - 69

25.
Let f(n) be the sum of the positive integer factors of n, where n is an integer. Find the sum of all positive integers n less than 1000 such that f(f(n) - n) = f(n).
A - 420, B - 530, C - 690, D - 911, E - 1034
1 reply
freddyfazbear
Today at 4:40 AM
WannabeUSAMOkid
Today at 5:21 AM
usamOOK geometry
KevinYang2.71   86
N Today at 4:37 AM by deduck
Source: USAMO 2025/4, USAJMO 2025/5
Let $H$ be the orthocenter of acute triangle $ABC$, let $F$ be the foot of the altitude from $C$ to $AB$, and let $P$ be the reflection of $H$ across $BC$. Suppose that the circumcircle of triangle $AFP$ intersects line $BC$ at two distinct points $X$ and $Y$. Prove that $C$ is the midpoint of $XY$.
86 replies
KevinYang2.71
Mar 21, 2025
deduck
Today at 4:37 AM
Scary Binomial Coefficient Sum
EpicBird08   38
N Today at 4:31 AM by Mathandski
Source: USAMO 2025/5
Determine, with proof, all positive integers $k$ such that $$\frac{1}{n+1} \sum_{i=0}^n \binom{n}{i}^k$$is an integer for every positive integer $n.$
38 replies
EpicBird08
Mar 21, 2025
Mathandski
Today at 4:31 AM
Good AIME/Olympiad Level Number Theory Books
MathRook7817   2
N Today at 4:09 AM by MathRook7817
Hey guys, do you guys have any good AIME/USAJMO Level Number Theory book suggestions?
I'm trying to get 10+ on next year's AIME and hopefully qual for USAJMO.
2 replies
MathRook7817
Today at 3:30 AM
MathRook7817
Today at 4:09 AM
[TEST RELEASED] Mock Geometry Test for College Competitions
Bluesoul   22
N Today at 3:32 AM by QuestionSourcer
Hi AOPSers,

I have finished writing a mock geometry test for fun and practice for the real college competitions like HMMT/PUMaC/CMIMC... There would be 10 questions and you should finish the test in 60 minutes, the test would be close to the actual test (hopefully). You could sign up under this thread, PM me your answers!. The submission would close on March 31st at 11:59PM PST.

I would create a private discussion forum so everyone could discuss after finishing the test. This is the first mock I've written, please sign up and enjoy geometry!!

~Bluesoul

Discussion forum: Discussion forum

Leaderboard
22 replies
Bluesoul
Feb 24, 2025
QuestionSourcer
Today at 3:32 AM
what the yap
KevinYang2.71   25
N Today at 3:24 AM by Mathandski
Source: USAMO 2025/3
Alice the architect and Bob the builder play a game. First, Alice chooses two points $P$ and $Q$ in the plane and a subset $\mathcal{S}$ of the plane, which are announced to Bob. Next, Bob marks infinitely many points in the plane, designating each a city. He may not place two cities within distance at most one unit of each other, and no three cities he places may be collinear. Finally, roads are constructed between the cities as follows: for each pair $A,\,B$ of cities, they are connected with a road along the line segment $AB$ if and only if the following condition holds:
[center]For every city $C$ distinct from $A$ and $B$, there exists $R\in\mathcal{S}$ such[/center]
[center]that $\triangle PQR$ is directly similar to either $\triangle ABC$ or $\triangle BAC$.[/center]
Alice wins the game if (i) the resulting roads allow for travel between any pair of cities via a finite sequence of roads and (ii) no two roads cross. Otherwise, Bob wins. Determine, with proof, which player has a winning strategy.

Note: $\triangle UVW$ is directly similar to $\triangle XYZ$ if there exists a sequence of rotations, translations, and dilations sending $U$ to $X$, $V$ to $Y$, and $W$ to $Z$.
25 replies
KevinYang2.71
Mar 20, 2025
Mathandski
Today at 3:24 AM
USACO US Open
neeyakkid23   20
N Today at 2:48 AM by aidan0626
Howd you all do?

Also will a 766 make bronze -> silver?
20 replies
neeyakkid23
Yesterday at 12:00 PM
aidan0626
Today at 2:48 AM
Elegant inequality
SunnyEvan   4
N Mar 23, 2025 by SunnyEvan
Source: proposed by Zhenping An
Let $a$, $b$, $c$, $d$ be non-negative real numbers such that
\[2a+2b+2c+2d+ab+bc+cd+da+3=abcd.\]prove that : \[\sqrt[4]{abc}+\sqrt[4]{bcd}+\sqrt[4]{cda}+\sqrt[4]{dab}\le\sqrt[4]{27(1+a)(1+b)(1+c)(1+d)}.\]
4 replies
SunnyEvan
Mar 22, 2025
SunnyEvan
Mar 23, 2025
Elegant inequality
G H J
G H BBookmark kLocked kLocked NReply
Source: proposed by Zhenping An
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SunnyEvan
39 posts
#1
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Let $a$, $b$, $c$, $d$ be non-negative real numbers such that
\[2a+2b+2c+2d+ab+bc+cd+da+3=abcd.\]prove that : \[\sqrt[4]{abc}+\sqrt[4]{bcd}+\sqrt[4]{cda}+\sqrt[4]{dab}\le\sqrt[4]{27(1+a)(1+b)(1+c)(1+d)}.\]
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SunnyEvan
39 posts
#2
Y by
who can help me :-D
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cube4320
3 posts
#3
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Not sure if it might help but: Click to reveal hidden text
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SunnyEvan
39 posts
#4
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no one ? :welcome:
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SunnyEvan
39 posts
#5
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$$ (a+a+a+3)(b+b+3+b)(c+3+c+c)(3+d+d+d) \geq (\sqrt[4]{3abc}+\sqrt[4]{3bcd}+\sqrt[4]{3cda}+\sqrt[4]{3dab})^4 $$==>$$ 81(a+1)(b+1)(c+1)(d+1) \geq 3(\sum \sqrt[4]{abc})^4 $$==>\[\sqrt[4]{abc}+\sqrt[4]{bcd}+\sqrt[4]{cda}+\sqrt[4]{dab}\le\sqrt[4]{27(1+a)(1+b)(1+c)(1+d)}.\]
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