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
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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.

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0 replies
jwelsh
Jul 1, 2025
0 replies
Stuck completely
littleduckysteve   2
N 2 minutes ago by littleduckysteve
I was wondering if anyone can solve this extremely hard question. I am stuck.

Let $S_n$ be the set of shapes with a regular polygon with sides of length $1$. Surrounded with triangles with sides $1$, $\sqrt{5}$, $\sqrt{5}$. Such that the part of the triangles with side length $1$ is on each of the sides, with the triangles outside of the regular polygon. Let $P_n$ be the probability that any two randomly chosen points in the shape $S_n$, have the line segment between them being totally contained inside of $S_n$, having no parts of the line located outside of the region $S_n$. What is the average of the values in the set, $(P_3,P_4,...P_{100})$. Write your answer as a fraction.

mock AMC 10 #25
2 replies
littleduckysteve
22 minutes ago
littleduckysteve
2 minutes ago
2018 preRMO p18, diopantine 4abc = (a+3)(b+3)(c+3), a+b+c =?
parmenides51   4
N 43 minutes ago by cortex_classes
If $a, b, c \ge 4$ are integers, not all equal, and $4abc = (a+3)(b+3)(c+3)$ then what is the value of $a+b+c$ ?
4 replies
parmenides51
Aug 7, 2019
cortex_classes
43 minutes ago
2018 preRMO p17 \A=\D, AB=DE=17, BC=EF=10,AC−DF=12, AC + DF ?
parmenides51   8
N 44 minutes ago by cortex_classes
Triangles $ABC$ and $DEF$ are such that $\angle A = \angle D, AB = DE = 17, BC = EF = 10$ and $AC - DF = 12$. What is $AC + DF$?
8 replies
parmenides51
Aug 8, 2019
cortex_classes
44 minutes ago
Interesting Summation
fjm30   21
N 44 minutes ago by cortex_classes
What is the value of $ { \sum_{1 \le i< j \le 10}(i+j)}_{i+j=odd} $ $ - { \sum_{1 \le i< j \le 10}(i+j)}_{i+j=even} $
21 replies
fjm30
Jun 8, 2019
cortex_classes
44 minutes ago
Inequalities
sqing   11
N an hour ago by DAVROS
Let $ a,b,c $ be real numbers . Prove that
$$- \frac{64(9+2\sqrt{21})}{9} \leq \frac {(ab-4)(bc-4)(ca-4) } {(a^2+a +1)(b^2+b +1)(c^2+c +1)}\leq \frac{16}{9}$$$$- \frac{8(436+79\sqrt{31})}{27} \leq  \frac {(ab-5)(bc-5)(ca-5) } {(a^2+a +1)(b^2+b +1)(c^2+c +1)}\leq \frac{25}{12}$$
11 replies
sqing
Jul 6, 2025
DAVROS
an hour ago
Airline Costs
AlcumusGuy   4
N an hour ago by littleduckysteve
Airline A charges $8$ cents per mile with an initial fee of $\$50$. Airline B charges $10$ cents per mile with an initial fee of $\$40$. How many miles must a flight be if it costs the same for both Airline A and Airline B?

$\textbf{(A) } 50 \qquad \textbf{(B) } 100 \qquad \textbf{(C) } 500 \qquad \textbf{(D) } 1000 \qquad \textbf{(E) } 5000$

(Mock AMC 10 #1)
4 replies
AlcumusGuy
Jan 15, 2015
littleduckysteve
an hour ago
2^m-n!=200 [original problem]
aaa12345   1
N an hour ago by nudinhtien
Find all ordered pairs of positive integers $(m,n)$ such that $2^m-n!=200.$
Answer
Solution
1 reply
aaa12345
2 hours ago
nudinhtien
an hour ago
Triangle Areas
AlcumusGuy   3
N an hour ago by littleduckysteve
Triangle $ABC$ has an area of $60$ with $BC = 9$. Point $D$ is on line $\overline{BC}$ with $CD = 6$. What is the positive difference between the two possible areas of triangle $ABD$?

$\textbf{(A) } 20 \qquad \textbf{(B) } 60 \qquad \textbf{(C) } 80 \qquad \textbf{(D) } 100 \qquad \textbf{(E) } 120$

(Mock AMC 10 #4)
3 replies
AlcumusGuy
Jan 15, 2015
littleduckysteve
an hour ago
k!+2023=n^2 [2024 Mathira]
aaa12345   0
2 hours ago
A positive integer $k$ is said to be $\textit{AMSLI-perfect}$ if $k!+2023$ is a perfect square. Find the sum of all $\textit{AMSLI-perfect}$ numbers.
Answer
Solution
Source: 2024 Mathirang Mathibay Orals/Tier 7-1A
0 replies
aaa12345
2 hours ago
0 replies
[20th PMO Qualifying Stage] 1.15
pensive   1
N 3 hours ago by pensive
Suppose that $\{a_n\}$ is a nonconstant arithmetic sequence such that $a_1 = 1$ and the terms $a_3, a_{15}, a_{24}$ form a geometric sequence in that order. Find the smallest index $n$ for which $a_n < 0$.

Answer
1 reply
pensive
3 hours ago
pensive
3 hours ago
Inequalities
sqing   27
N 3 hours ago by DAVROS
Let $ x,y,z $ be real numbers such that $ x^2+2y^2+z^2+xy+yz+zx=1 $.Prove that$$-\sqrt{\frac 75}\leq  x+y+z   \leq  \sqrt{\frac 75}$$
27 replies
sqing
Aug 31, 2024
DAVROS
3 hours ago
Padres vs Dodgers expected wins
ehz2701   0
5 hours ago
Suppose the Padres have 6 pitchers of skill level $\{2, 4, 6, 8, 10, 12\}$. The dodgers have 6 pitchers of skill level $\{1,3,5,7,9, 11\}$. When a Padres pitcher with skill level $a$ is against a Dodgers pitcher with skill level $b$, the probability that the Padres win that game is $\frac{a}{a+b}$. The Padres have already determined that they will start their pitchers in the increasing order of skill level. In what order should the dodgers start their pitchers to maximize their expected wins? Give your answer as a sequence of six integers.

***DISCLAIMER: (1) If you are a hitter, I know you are very vital and important to the game, and pitching does not determine everything. (2) Yes, there is no such thing as a one-all stat like skill level. ERA, WHIP, FIP, and many other stats govern just fine. WAR even. (3) I am NOT making a statement on whether the dodgers and padres pitching staff is better.
0 replies
ehz2701
5 hours ago
0 replies
(a+sqrt(b)^p when a-sqrt(b) is actually larger than 1
ehz2701   0
5 hours ago
We’ve all seen problems like what is the one’s place of $(\sqrt{2}+1)^{2025}$? (It’s 5 btw). In any such radical $a+\sqrt{b}$ for some integers $a,b$, the key is that $a-\sqrt{b} < 1$ such that $(a-\sqrt{b})^{p}$ is negligibly small. And adding this to $(a+\sqrt{b})^{p}$ squares away the square roots and cancels away the roots bits, leaving only integers behind. From this answer extraction is typically easy, perhaps a little module arithmetic is needed in combination with the binomial theorem.

Now, what if the conjugate $a-\sqrt{b}>1$? For example, what is the ones digit of $(3+\sqrt{2})^{2025}$? Is this feasible? answer according to wolfram alpha
0 replies
ehz2701
5 hours ago
0 replies
2002 TAMU - Best Student Exam - Texas A&M University HS Mathematics Contest
parmenides51   10
N Today at 4:13 AM by imtiyas1
p1. The product of two numbers is $7$ and the sum of their reciprocals is $4$. What is the sum of these two numbers?


p2. Circles are both inscribed within and circumscribed about an equilateral triangle. The length of a side of the triangle is $4$. What is the ratio of the area of the circumscribed circle to the area of the inscribed circle?


p3. If $(x - a)(x - b)(x - c)(x - d)(x - e) = x^5 -7x^4 + 6x^3 + 5x^4 + 13x-9$ , then $a + b + c + d + e = ...$


p4. Find the area above the $x$-axis that is enclosed by the graph of the curve $f(x) = 1-2\left| x- \frac12\right|$.


p5. Let $g$ be a function that satisfies the following properties:
(i) $g(0) = 2$ .
(ii) $g(1) = 3$.
(iii) $g(x + y) + g(x-y) = g(x)g(y)$ for all integers $x$ and $y$ .
Find $g(5)$ .


p6. The center of a circle of radius $4$ is located at the center of a square table with side $16$. A coin with radius $1/8$ is randomly thrown onto the table. What is the probability that the coin comes to rest on the boundary of the circle?


p7. Find the value of
$$\frac{3}{(1 \cdot 2)^2} + \frac{5}{(2 \cdot 3)^2} +\frac{7}{(3 \cdot 4)^2} +\frac{9}{(4 \cdot 5)^2} + ...+\frac{2001}{
(1000 \cdot 1001)^2}$$

p8. Two candles of equal length start burning at the same time. One of the candles will burn in $4$ hours, and the other in $5$ hours. How long in hours will they have to burn before one candle is $3$ times the length of the other?


p9. Three integers form a geometric progression. Their sum is $21$ and the sum of their reciprocals is $\frac{7}{12}$. Find the largest integer.


p10. There are eight men in a room. Each one shakes hands with each of the others once. How many handshakes are there?


p11. Two squares (shown below), each with side $12$, are placed so that a corner of one lies at the center of the other. Find the area of quadrilateral $EJCK$ if $BJ= 4$.
IMAGE


p12. In a $10$-team baseball league, each team plays each of the others $18$ times. The season ends, not in a tie, with each team the same number of games ahead of the following team. What is the greatest number of games that the last team could have won?


p13. Find the unique pair of real numbers $(x, y)$ such that $(4x^2 + 6x + 4)(4y^2 - 12y + 25) = 28$ .


p14. A man and his grandson have the same birthday. For six consecutive birthdays the man is an integral number of times as old as his grandson. How old is the man at the sixth of these birthdays?


p15. In the product $9 \cdot HATBOX = 4  \cdot BOXHAT$ , find the six-digit number $BOXHAT$ .


p16. If $n$ is an even integer, express in terms of $n$ the number of solutions in positive integers of $2x+y+z = n$ .


p17. A sequence is defined by $x_1 = 2$ and $x_{n+1} =\frac{x_n}{1+ x_n}$ for all $n \ge 1$ . Find $x_{10,000 }$.


p18. If $a =\frac{x}{x^2 + y^2}$ and $b =\frac{y}{x^2 + y^2}$ , find $x + y$ in terms of $a$ and $b$ . Express your answer as a common fraction.




PS. You should use hide for answers. Collected here.
10 replies
parmenides51
Mar 23, 2022
imtiyas1
Today at 4:13 AM
Trapezium problem very nice
manlio   4
N May 18, 2025 by alexheinis
Given trapezium ABCD with basis AB and CD parallel. Choose a point E on side BC and a point F on side AD such that AE Is parallel to FC . Prove that DE Is parallel to FB.
4 replies
manlio
May 17, 2025
alexheinis
May 18, 2025
Trapezium problem very nice
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manlio
3255 posts
#1 • 1 Y
Y by PikaPika999
Given trapezium ABCD with basis AB and CD parallel. Choose a point E on side BC and a point F on side AD such that AE Is parallel to FC . Prove that DE Is parallel to FB.
This post has been edited 3 times. Last edited by manlio, May 17, 2025, 12:54 PM
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vanstraelen
9153 posts
#2 • 1 Y
Y by PikaPika999
Given the trapezoid $ABCD\ :\ A(0,0),B(b,0),C(c,d),D(a,d)$.

Choose $F(\lambda,\frac{d\lambda}{a})$, then $E(\frac{ab(c-\lambda)}{\lambda (c-b-a)+ab},\frac{bd(a-\lambda)}{\lambda (c-b-a)+ab})$.

Slope of the line $BF\ :\ m_{BF}=\frac{d\lambda}{a(\lambda-b)}=m_{DE}$, the slope of the line $DE$.
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alexheinis
10732 posts
#3 • 1 Y
Y by Bekzhankam
Let's suppose $CD<AB$, then we have an intersection $S$ as in the picture.
We have ${{SD}\over {SC}}={{SA}\over {SB}}, {{SF}\over {SA}}={{SC}\over {SE}}$ hence $SB\cdot SD=SE\cdot SF\implies {{SD}\over {SF}}={{SE}\over {SB}}\implies DE//BF$.
A similar argument holds when $AB<DC$.
If $AB=CD$ then $ABCD$ is a pgm and $AECF$ is a pgm. Hence $AF=CE\implies DF=BE\implies DE//BF$.
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sunken rock
4407 posts
#4
Y by
alexheinis wrote:
Let's suppose $CD<AB$, then we have an intersection $S$ as in the picture.
We have ${{SD}\over {SC}}={{SA}\over {SB}}, {{SF}\over {SA}}={{SC}\over {SE}}$ hence $SB\cdot SD=SE\cdot SF\implies {{SD}\over {SF}}={{SE}\over {SB}}\implies DE//BF$.
A similar argument holds when $AB<DC$.
If $AB=CD$ then $ABCD$ is a pgm and $AECF$ is a pgm. Hence $AF=CE\implies DF=BE\implies DE//BF$.

If... then $ABCD, AECF$ are isosceles trapezoids, otherwise OK!
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alexheinis
10732 posts
#5
Y by
@sunkenrock: I did mean parallellogram. The parallel sides of $ABCD$ are given to be equal, not the upstanding sides. See the picture below for clarification.
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