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k a July Highlights and 2025 AoPS Online Class Information
jwelsh   0
Jul 1, 2025
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0 replies
1 viewing
jwelsh
Jul 1, 2025
0 replies
Nt with casework
Tofa7a._.36   1
N 15 minutes ago by aaravdodhia
Find all nonnegative integers \( x, y \in \mathbb{N} \) and primes \( p \) such that:
\[
x^2 = p - 2 \qquad \text{and} \qquad y^2 = 2p^2 - 2
\]
1 reply
Tofa7a._.36
2 hours ago
aaravdodhia
15 minutes ago
10 Problems
Sedro   16
N 30 minutes ago by Sedro
Title says most of it. I've been meaning to post a problem set on HSM since at least a few months ago, but since I proposed the most recent problems I made to the 2025 SSMO, I had to wait for that happen. (Hence, most of these problems will probably be familiar if you participated in that contest, though numbers and wording may be changed.) The problems are very roughly arranged by difficulty. Enjoy!

Problem 1: An sequence of positive integers $u_1, u_2, \dots, u_8$ has the property for every positive integer $n\le 8$, its $n^\text{th}$ term is greater than the mean of the first $n-1$ terms, and the sum of its first $n$ terms is a multiple of $n$. Let $S$ be the number of such sequences satisfying $u_1+u_2+\cdots + u_8 = 144$. Compute the remainder when $S$ is divided by $1000$.

Problem 2 (solved by fruitmonster97): Rhombus $PQRS$ has side length $3$. Point $X$ lies on segment $PR$ such that line $QX$ is perpendicular to line $PS$. Given that $QX=2$, the area of $PQRS$ can be expressed as $\tfrac{m}{n}$, where $m$ and $n$ are relatively prime positive integers. Compute $m+n$.

Problem 3: Positive integers $a$ and $b$ satisfy $a\mid b^2$, $b\mid a^3$, and $a^3b^2 \mid 2025^{36}$. If the number of possible ordered pairs $(a,b)$ is equal to $N$, compute the remainder when $N$ is divided by $1000$.

Problem 4: Let $ABC$ be a triangle. Point $P$ lies on side $BC$, point $Q$ lies on side $AB$, and point $R$ lies on side $AC$ such that $PQ=BQ$, $CR=PR$, and $\angle APB<90^\circ$. Let $H$ be the foot of the altitude from $A$ to $BC$. Given that $BP=3$, $CP=5$, and $[AQPR] = \tfrac{3}{7} \cdot [ABC]$, the value of $BH\cdot CH$ can be expressed as $\tfrac{m}{n}$, where $m$ and $n$ are relatively prime positive integers. Compute $m+n$.

Problem 5 (solved by maromex): Anna has a three-term arithmetic sequence of integers. She divides each term of her sequence by a positive integer $n>1$ and tells Bob that the three resulting remainders are $20$, $52$, and $R$, in some order. For how many values of $R$ is it possible for Bob to uniquely determine $n$?

Problem 6: There is a unique ordered triple of positive reals $(x,y,z)$ satisfying the system of equations \begin{align*} x^2 + 9 &= (y-\sqrt{192})^2 + 4 \\ y^2 + 4 &= (z-\sqrt{192})^2 + 49 \\ z^2 + 49 &= (x-\sqrt{192})^2 + 9. \end{align*}The value of $100x+10y+z$ can be expressed as $p\sqrt{q}$, where $p$ and $q$ are positive integers such that $q$ is square-free. Compute $p+q$.

Problem 7: Let $S$ be the set of all monotonically increasing six-term sequences whose terms are all integers between $0$ and $6$ inclusive. We say a sequence $s=n_1, n_2, \dots, n_6$ in $S$ is symmetric if for every integer $1\le i \le 6$, the number of terms of $s$ that are at least $i$ is $n_{7-i}$. The probability that a randomly chosen element of $S$ is symmetric is $\tfrac{p}{q}$, where $p$ and $q$ are relatively prime positive integers. Compute $p+q$.

Problem 8: For a positive integer $n$, let $r(n)$ denote the value of the binary number obtained by reading the binary representation of $n$ from right to left. Find the smallest positive integer $k$ such that the equation $n+r(n)=2k$ has at least ten positive integer solutions $n$.

Problem 9: Let $p$ be a quadratic polynomial with a positive leading coefficient. There exists a positive real number $r$ such that $r < 1 < \tfrac{5}{2r} < 5$ and $p(p(x)) = x$ for $x \in \{ r,1,  \tfrac{5}{2r} , 5\}$. Compute $p(20)$.

Problem 10: Find the number of ordered triples of positive integers $(a,b,c)$ such that $a+b+c=995$ and $ab+bc+ca$ is a multiple of $995$.
16 replies
Sedro
Jul 10, 2025
Sedro
30 minutes ago
Tangent at the orthocenter
Tofa7a._.36   0
2 hours ago
Let $ABC$ be a triangle. Consider the points $D,E,F$ as the feet of the altitudes from $A,B,C,$ respectively, and $H$ its orthocenter which we suppose is the midpoint of $CF$. Let $M$ be the midpoint of $BC$, $N$ be the midpoint of $BE$, and $X=(AN)\cap(MF).$
Prove that $\angle HXM=90^\circ$.
0 replies
Tofa7a._.36
2 hours ago
0 replies
Nt and chains
Tofa7a._.36   0
2 hours ago
Let \( a_0, a_1, \ldots, a_n \) be positive divisors of the number \( 2024^{2025} \) such that:

\(\bullet\) \( a_0 < a_1 < a_2 < \cdots < a_n \)

\(\bullet\) \( a_0 \mid a_1,\, a_1 \mid a_2,\, \ldots,\, a_{n-1} \mid a_n \)

Find the largest possible value of the positive integer \( n \).
0 replies
Tofa7a._.36
2 hours ago
0 replies
No more topics!
Square Pattern
4everwise   22
N Jun 14, 2025 by JH_K2IMO
Figures $ 0$, $ 1$, $ 2$, and $ 3$ consist of $ 1$, $ 5$, $ 13$, and $ 25$ nonoverlapping squares, respectively. If the pattern were continued, how many nonoverlapping squares would there be in figure $ 100$?
IMAGE$ \textbf{(A)}\ 10401 \qquad \textbf{(B)}\ 19801 \qquad \textbf{(C)}\ 20201 \qquad \textbf{(D)}\ 39801 \qquad \textbf{(E)}\ 40801$
22 replies
4everwise
Jan 24, 2006
JH_K2IMO
Jun 14, 2025
Square Pattern
G H J
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4everwise
2532 posts
#1 • 5 Y
Y by Adventure10, jhu08, Mango247, Ravensrule8, Panda729
Figures $ 0$, $ 1$, $ 2$, and $ 3$ consist of $ 1$, $ 5$, $ 13$, and $ 25$ nonoverlapping squares, respectively. If the pattern were continued, how many nonoverlapping squares would there be in figure $ 100$?
[asy]
unitsize(8);
draw((0,0)--(1,0)--(1,1)--(0,1)--cycle);
draw((9,0)--(10,0)--(10,3)--(9,3)--cycle);
draw((8,1)--(11,1)--(11,2)--(8,2)--cycle);
draw((19,0)--(20,0)--(20,5)--(19,5)--cycle);
draw((18,1)--(21,1)--(21,4)--(18,4)--cycle);
draw((17,2)--(22,2)--(22,3)--(17,3)--cycle);
draw((32,0)--(33,0)--(33,7)--(32,7)--cycle);
draw((29,3)--(36,3)--(36,4)--(29,4)--cycle);
draw((31,1)--(34,1)--(34,6)--(31,6)--cycle);
draw((30,2)--(35,2)--(35,5)--(30,5)--cycle);
label("Figure",(0.5,-1),S);
label("$0$",(0.5,-2.5),S);
label("Figure",(9.5,-1),S);
label("$1$",(9.5,-2.5),S);
label("Figure",(19.5,-1),S);
label("$2$",(19.5,-2.5),S);
label("Figure",(32.5,-1),S);
label("$3$",(32.5,-2.5),S);[/asy]$ \textbf{(A)}\ 10401 \qquad \textbf{(B)}\ 19801 \qquad \textbf{(C)}\ 20201 \qquad \textbf{(D)}\ 39801 \qquad \textbf{(E)}\ 40801$
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4everwise
2532 posts
#2 • 6 Y
Y by Adventure10, jhu08, Mango247, Ravensrule8, Panda729, and 1 other user
Solution
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LordoftheMorons
493 posts
#3 • 3 Y
Y by Adventure10, jhu08, Math5Rock5
Whats with the broken lines in figure 3?
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236factorial
4555 posts
#4 • 3 Y
Y by smoothtorus, Adventure10, jhu08
I needed some polynomial work.

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boxedexe
2012 posts
#5 • 2 Y
Y by Adventure10, jhu08
This is quite simple. The left and right side squares add up to $2n^2$ squares (Note: $\sum_{i=1}^n(2i-1)=n^2$). The middle column has $2n+1$ squares. Therefore, we add all the squares and get $f(n)=2n^2+2n+1$. It could also be done using area in terms of the $n$th figure.

EDIT: I had an x in f instead of an n. :P
Masoud Zargar
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now a ranger
1194 posts
#6 • 3 Y
Y by Adventure10, jhu08, Mango247
How did you derive the polynomial equation?

Is this combinatorics?
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stupidityismygam
150 posts
#7 • 2 Y
Y by jhu08, Adventure10
now a ranger wrote:
How did you derive the polynomial equation?

Is this combinatorics?

Alright you have, $f(0)=1, f(1)=5, f(2)=13, f(3)=25$

This is a quadratic because all the second differences are the same ($f(1)-f(0)=4, f(2)-f(1)=8, f(3)-f(2)=12$ are the first differences, then $8-4, 12-8$ are the second differences, since these are the same it is a quadratic). Quadratics are of the form $ax^{2}+bx+c$. So, plug in $0, 1, 2, 3$ in for x, and solve for $a,b,c$. This gives you $f(x)=2x^{2}+2x+1$
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cincodemayo5590
1117 posts
#8 • 3 Y
Y by Adventure10, jhu08, Mango247
And if you wanted to be nitpicking, you would have to prove that this quadratic function works via induction, boxedexe's method, or a variety of other ways. On a written proof, I wouldn't just use the finite differences without a proof that the formula works since you don't know if the next values will agree or not; but on a contest where only the answer is required (AMC, AIME, etc) it'd be perfectly fine--whatever saves you time is the best method.
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gangsines
20 posts
#9 • 2 Y
Y by jhu08, Adventure10
quadratic sequences, right?
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m5s
12 posts
#10 • 3 Y
Y by jhu08, Adventure10, Mango247
here's a simple, intuitive way to find a formula for this sequence:
Click to reveal hidden text
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talkinaway
5083 posts
#11 • 2 Y
Y by jhu08, Adventure10
Visual PIE
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AKAL3
753 posts
#12 • 3 Y
Y by jhu08, Adventure10, Mango247
Another View
Attachments:
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Hu_bryan
2 posts
#13 • 3 Y
Y by jhu08, Adventure10, Mango247
I know it is not necessary, but one can also use differential equations

As the figure n increases, the number of squares being added on each time is increasing by 4. Thus we can say:

y' = 4n + 1
dy/dn = 4n + 1
∫dy = ∫(4n + 1) dn
y = 2n^2 + n + C // C in this situation is 1, the number of squares in figure 0
y = 2n^2 + n + 1 // Plug in 100 for n
y = 20101
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jwhouston
113 posts
#14 • 3 Y
Y by jhu08, Adventure10, Mango247
Click to reveal hidden text
EDIT: Oops, I just realized that other people already posted this, only $x^2+(x+1)^2$ was simplified to $2x^2+2x+1.$
This post has been edited 1 time. Last edited by jwhouston, Jun 21, 2019, 5:10 PM
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space-2
789 posts
#15 • 1 Y
Y by Adventure10
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ln2rv
1 post
#16 • 2 Y
Y by Adventure10, Mango247
I had a kind of longer solution.

Click to reveal hidden text
This post has been edited 1 time. Last edited by ln2rv, Dec 14, 2019, 10:01 PM
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Imayormaynotknowcalculus
974 posts
#17 • 2 Y
Y by Adventure10, Mango247
4everwise wrote:
Figures $ 0$, $ 1$, $ 2$, and $ 3$ consist of $ 1$, $ 5$, $ 13$, and $ 25$ nonoverlapping squares, respectively. If the pattern were continued, how many nonoverlapping squares would there be in figure $ 100$?
[asy]
unitsize(8);
draw((0,0)--(1,0)--(1,1)--(0,1)--cycle);
draw((9,0)--(10,0)--(10,3)--(9,3)--cycle);
draw((8,1)--(11,1)--(11,2)--(8,2)--cycle);
draw((19,0)--(20,0)--(20,5)--(19,5)--cycle);
draw((18,1)--(21,1)--(21,4)--(18,4)--cycle);
draw((17,2)--(22,2)--(22,3)--(17,3)--cycle);
draw((32,0)--(33,0)--(33,7)--(32,7)--cycle);
draw((29,3)--(36,3)--(36,4)--(29,4)--cycle);
draw((31,1)--(34,1)--(34,6)--(31,6)--cycle);
draw((30,2)--(35,2)--(35,5)--(30,5)--cycle);
label("Figure",(0.5,-1),S);
label("$0$",(0.5,-2.5),S);
label("Figure",(9.5,-1),S);
label("$1$",(9.5,-2.5),S);
label("Figure",(19.5,-1),S);
label("$2$",(19.5,-2.5),S);
label("Figure",(32.5,-1),S);
label("$3$",(32.5,-2.5),S);[/asy]$ \textbf{(A)}\ 10401 \qquad \textbf{(B)}\ 19801 \qquad \textbf{(C)}\ 20201 \qquad \textbf{(D)}\ 39801 \qquad \textbf{(E)}\ 40801$

Count the diagonal rows of each figure. Notice that in each figure $n$, we count the squares with rows of length $n$ and $n+1$.
Then, we have a total of $n^2+(n+1)^2=2n^2+2n+1$ squares. Plug in $n=100$ to obtain $20000+200+1=20201$ squares in figure $100$.
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Imayormaynotknowcalculus
974 posts
#18 • 1 Y
Y by Adventure10
Hu_bryan wrote:
I know it is not necessary, but one can also use differential equations

As the figure n increases, the number of squares being added on each time is increasing by 4. Thus we can say:

y' = 4n + 1
dy/dn = 4n + 1
∫dy = ∫(4n + 1) dn
y = 2n^2 + n + C // C in this situation is 1, the number of squares in figure 0
y = 2n^2 + n + 1 // Plug in 100 for n
y = 20101

Actually, using differential equations doesn't work; the integral represents integrating over a continuous interval, not over increments.
If you were to set up a differential equation for the continuous version of the function, you would have $y'=4n$ in a continuous form and integrating would yield $y=2n^2+C$.
You could use an initial condition of $y(0)=1$ to let $C=1$.
This yields an approximation of the result, as $2(100^2)+1$ is closest to $B$ and $C$.
This can be resolved by noting that the function under-approximates the original incremental approach.
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OlympusHero
17020 posts
#20
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Solution
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TanMatrix
1 post
#21
Y by
In first its 1,5,13...,so as i observed its increasing in format of 1+1(4)+2(4)............n(4).
so for 100 it will be 1+4+8+16....400 so simply leaving 1 is coming an A.P. series so we will apply the sum of an A.P. formula - n/2(1st term + last term ) in 4+8+16...400
which is coming as 20200 the add the 1 so its 20201 (C).
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Husnain_Haider
1 post
#22
Y by
It is quite simple.

ans = 20201
general form = 2n^2 + 2n + 1
solution :
* Ignoring the line in mid first.
* As we know that sum of first n odd numbers if n^2
* Hence, n^2 + n^2 = 2n^2
* Now as we know that the line in mid is 2n+1
so : 2n^2 + 2n + 1
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Mingminghu
227 posts
#23
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JH_K2IMO
156 posts
#24
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(Figure 100)
(1+3+5+...+199)*2 +201
=(100*100)*2 +201= 20201
The answer is (C)

(Figure n)
(1+3+5+...+(2n-1))*2 +(2n+1)
=n*n*2 +(2n+1)
2n^2+2n+1.
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