Plan ahead for the next school year. Schedule your class today!

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k a July Highlights and 2025 AoPS Online Class Information
jwelsh   0
Jul 1, 2025
We are halfway through summer, so be sure to carve out some time to keep your skills sharp and explore challenging topics at AoPS Online and our AoPS Academies (including the Virtual Campus)!

[list][*]Over 60 summer classes are starting at the Virtual Campus on July 7th - check out the math and language arts options for middle through high school levels.
[*]At AoPS Online, we have accelerated sections where you can complete a course in half the time by meeting twice/week instead of once/week, starting on July 8th:
[list][*]MATHCOUNTS/AMC 8 Basics
[*]MATHCOUNTS/AMC 8 Advanced
[*]AMC Problem Series[/list]
[*]Plus, AoPS Online has a special seminar July 14 - 17 that is outside the standard fare: Paradoxes and Infinity
[*]We are expanding our in-person AoPS Academy locations - are you looking for a strong community of problem solvers, exemplary instruction, and math and language arts options? Look to see if we have a location near you and enroll in summer camps or academic year classes today! New locations include campuses in California, Georgia, New York, Illinois, and Oregon and more coming soon![/list]

MOP (Math Olympiad Summer Program) just ended and the IMO (International Mathematical Olympiad) is right around the corner! This year’s IMO will be held in Australia, July 10th - 20th. Congratulations to all the MOP students for reaching this incredible level and best of luck to all selected to represent their countries at this year’s IMO! Did you know that, in the last 10 years, 59 USA International Math Olympiad team members have medaled and have taken over 360 AoPS Online courses. Take advantage of our Worldwide Online Olympiad Training (WOOT) courses
and train with the best! Please note that early bird pricing ends August 19th!
Are you tired of the heat and thinking about Fall? You can plan your Fall schedule now with classes at either AoPS Online, AoPS Academy Virtual Campus, or one of our AoPS Academies around the US.

Our full course list for upcoming classes is below:
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0 replies
jwelsh
Jul 1, 2025
0 replies
Trigonometry equation practice
ehz2701   8
N 20 minutes ago by vanstraelen
There is a lack of trigonometric bash practice, and I want to see techniques to do these problems. So here are 10 kinds of problems that are usually out in the wild. How do you tackle these problems? (I had ChatGPT write a code for this.). Please give me some general techniques to solve these kinds of problems, especially set 2b. I’ll add more later.

Leaderboard

problem set 1a

problem set 2a

problem set 2b
answers 2b

General techniques so far:

Trick 1: one thing to keep in mind is

[center] $\frac{1}{2} = \cos 36 - \sin 18$. [/center]

Many of these seem to be reducible to this. The half can be written as $\cos 60 = \sin 30$, and $\cos 36 = \sin 54$, $\sin 18 = \cos 72$. This is proven in solution 1a-1. We will refer to this as Trick 1.
8 replies
ehz2701
Jul 12, 2025
vanstraelen
20 minutes ago
Permutations of n-degree
red_dog   1
N 40 minutes ago by alexheinis
Let $S_n$ be the set of permutations of n-degree and $\sigma\in S_n, \ n\ge 3$. Prove that if $\sigma\alpha=\alpha\sigma, \ \forall \alpha\in S_n$, then $\sigma=e$ (where $e$ is the identical permutation).
1 reply
red_dog
an hour ago
alexheinis
40 minutes ago
Subgroups
red_dog   1
N an hour ago by alexheinis
Let $G_1,G_2,\ldots,G_{2002}$ be subgroups of the group $(\mathbb{Q},+)$ and $\mathbb{Q}=G_1\cup G_2\cup\ldots\cup G_{2002}$. Prove that exists $i\in\{1,2,\ldots,2002\}$ such as $G_i=\mathbb{Q}$.
1 reply
red_dog
2 hours ago
alexheinis
an hour ago
Challenge: Make as many positive integers from 2 zeros
Biglion   24
N 2 hours ago by littleduckysteve
How many positive integers can you make from at most 2 zeros, any math operation and cocatination?
New Rule: The successor function can only be used at most 3 times per number
Starting from 0, 0=0
24 replies
Biglion
Jul 2, 2025
littleduckysteve
2 hours ago
Modular Equality with Coprime Integers
pandev3   5
N 3 hours ago by pandev3
Let $(a)_n = a - n \left\lfloor \frac{a}{n} \right\rfloor$, which is equivalent to $a \mod n$ for any integer $a$.

Let $a_1, a_2, a_3, a_4$ and $n$ be positive integers such that $a_i$ is coprime with $n$ for $i = 1, 2, 3, 4$.

It holds that $(k a_1)_n + (k a_2)_n + (k a_3)_n + (k a_4)_n = 2n$ for $k = 1, 2, \dots, n - 1$.

Prove that $(a_1)_n + (a_j)_n = n$ for some $j$ where $2 \leq j \leq 4$.
5 replies
pandev3
Nov 21, 2024
pandev3
3 hours ago
Concurrency of Lines Involving Altitudes and Circumcenters in a Triangle
JackMinhHieu   0
3 hours ago
Hi everyone,
I recently came across an interesting geometry problem that I'd like to share. It involves a triangle inscribed in a circle, altitudes, points on arcs, and a surprising concurrency involving circumcenters. Here's the problem:
Problem:
Let ABC be an acute triangle inscribed in a circle (O). Let the altitudes AD, BE, CF intersect at the orthocenter H.
Points M and N lie on the minor arcs AB and AC of circle (O), respectively, such that MN // AB.
Let I be the circumcenter of triangle NEC, and J be the circumcenter of triangle MFB.
Prove that the lines OD, BI, and CJ are concurrent.

I find the configuration quite elegant, and I'm looking for different ways to approach the problem — whether it's synthetic, coordinate, vector-based, or inversion.
Any ideas, hints, or full solutions are appreciated. Thank you!
0 replies
JackMinhHieu
3 hours ago
0 replies
question
duckShroom   8
N 3 hours ago by littleduckysteve
Hi everyone,

So I'll give you the short version:

- My school's math program is nearly nonexistent

- The highest freshman course is a 2 year long course combining geometry, alg 2, and precalc. (Most people are struggling and drop out, which is because of the horrible orginaization and the fact they moved middle school back a year so the highest middle school course is alg 1)

- In the first year, we essentially skipped geometry and they'll most likely butcher precalc too

- I'm either the best or second best math student in my year and the year above me, and the school knows. Even so, they won't let me skip the 2nd year of the course I'm in to do AP Calc BC

- I know bits and pieces of precalc from comp stuff, but I need to learn standardized precalc on a deep level out of school without spending money. I know alg 2 well (besides some trig stuff) and geo, well, I haven't actually formally learnt it but I basically know it from the school's 'teachings', khan, and comp stuff.

- I also am reading a calc textbook to learn overall concepts and some basic derivates and all. (https://calculusmadeeasy.org/)

So basically, I need a good free online precalc course or textbook. Thanks for listening to my half rant and half question post thing. All help will be appreciated!

P.S: pm me if you want to be in a AIME study forum
8 replies
duckShroom
Jan 7, 2025
littleduckysteve
3 hours ago
2018 preRMO p22, k=sum of no in partition of {1, 2, ..., 20}, good integer
parmenides51   8
N 4 hours ago by cortex_classes
A positive integer $k$ is said to be good if there exists a partition of $ \{1, 2, 3,..., 20\}$ into disjoint proper subsets such that the sum of the numbers in each subset of the partition is $k$. How many good numbers are there?
8 replies
parmenides51
Aug 8, 2019
cortex_classes
4 hours ago
2018 preRMO p20, product of whose digits equals n^2 -15n -27
parmenides51   11
N 4 hours ago by cortex_classes
Determine the sum of all possible positive integers $n, $ the product of whose digits equals $n^2 -15n -27$.
11 replies
parmenides51
Aug 8, 2019
cortex_classes
4 hours ago
Infinite 6's
BlackPanther007   23
N 4 hours ago by cortex_classes
Let $N=6+66+666+....+666..66$, where there are hundred $6's$ in the last term in the sum. How many times does the digit $7$ occur in the number $N$
23 replies
BlackPanther007
Aug 19, 2018
cortex_classes
4 hours ago
Question about Notation
Quaratinium   4
N 4 hours ago by aidan0626
How are we suppose to know if something like $f^5(x)$ wants the 5th derivative, raise the function to the 5th power, or plug the thing back in the function five times?
4 replies
Quaratinium
Jun 28, 2018
aidan0626
4 hours ago
P-Adic MathDash Problem
LilKirb   2
N 5 hours ago by cheltstudent
Let $N = 2^{23} - 36.$ Given that $2^{21} - 9$ is a prime, find the number of nonnegative integers $0 \leq x \leq N$ such that $N$ is a divisor of $\binom{N}{x}.$

Express the answer in the form $a \cdot b^c - d,$ where $a,b,c,d$ are positive integers, $a$ is not divisible by $b,$ and $b$ is as small as possible with $b\neq1$
2 replies
LilKirb
Yesterday at 12:24 PM
cheltstudent
5 hours ago
Just for fun
Alexyuan2017   5
N 5 hours ago by Quadratic_rush
What is the derivative of y=x. see if you can figure it out without knowing calculus :D
5 replies
Alexyuan2017
May 6, 2023
Quadratic_rush
5 hours ago
Calculus-flavored sum
NamelyOrange   4
N 5 hours ago by Quadratic_rush
Evaluate $\sum_{n = 0}^{\infty}\frac{1}{(4n)!}$.

(Source: AOPS user @SomeDumbChild)
4 replies
NamelyOrange
Jun 24, 2025
Quadratic_rush
5 hours ago
Convex function trouble
Sedro   2
N Jun 17, 2024 by Sedro
Suppose a function $f$ satisfies $f(x)+f(y) \ge 2f(\tfrac{x+y}{2})$ for all real $x,y$. Then, we have \begin{align*} f(x)+f(y) &\ge 2f(\tfrac{x+y}{2}) \\ f(x)+f(-y) &\ge 2f(\tfrac{x-y}{2}). \end{align*}We also have \[f(\tfrac{x+y}{2}) + f(\tfrac{x-y}{2}) \ge 2f(x).\]Adding the first two inequalities and using the third, we have \[2f(x) + f(y)+f(-y) \ge 2(f(\tfrac{x+y}{2}) + f(\tfrac{x-y}{2})) \ge 4f(x),\]which implies $f(y)+f(-y) \ge 2f(x)$. But this means $f$ is bounded above which is absurd -- take the identity function. I feel I am making some blithely stupid mistake, but I can't see what. Any help is appreciated :)
2 replies
Sedro
Jun 17, 2024
Sedro
Jun 17, 2024
Convex function trouble
G H J
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Sedro
5882 posts
#1
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Suppose a function $f$ satisfies $f(x)+f(y) \ge 2f(\tfrac{x+y}{2})$ for all real $x,y$. Then, we have \begin{align*} f(x)+f(y) &\ge 2f(\tfrac{x+y}{2}) \\ f(x)+f(-y) &\ge 2f(\tfrac{x-y}{2}). \end{align*}We also have \[f(\tfrac{x+y}{2}) + f(\tfrac{x-y}{2}) \ge 2f(x).\]Adding the first two inequalities and using the third, we have \[2f(x) + f(y)+f(-y) \ge 2(f(\tfrac{x+y}{2}) + f(\tfrac{x-y}{2})) \ge 4f(x),\]which implies $f(y)+f(-y) \ge 2f(x)$. But this means $f$ is bounded above which is absurd -- take the identity function. I feel I am making some blithely stupid mistake, but I can't see what. Any help is appreciated :)
This post has been edited 3 times. Last edited by Sedro, Jun 17, 2024, 3:57 PM
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natmath
8219 posts
#2 • 1 Y
Y by Sedro
Your third inequality should be $2f(x/2)$ on the RHS.
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Sedro
5882 posts
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natmath wrote:
Your third inequality should be $2f(x/2)$ on the RHS.

:wallbash_red: Thanks.
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