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k a April Highlights and 2025 AoPS Online Class Information
jlacosta   0
Apr 2, 2025
Spring is in full swing and summer is right around the corner, what are your plans? At AoPS Online our schedule has new classes starting now through July, so be sure to keep your skills sharp and be prepared for the Fall school year! Check out the schedule of upcoming classes below.

WOOT early bird pricing is in effect, don’t miss out! If you took MathWOOT Level 2 last year, no worries, it is all new problems this year! Our Worldwide Online Olympiad Training program is for high school level competitors. AoPS designed these courses to help our top students get the deep focus they need to succeed in their specific competition goals. Check out the details at this link for all our WOOT programs in math, computer science, chemistry, and physics.

Looking for summer camps in math and language arts? Be sure to check out the video-based summer camps offered at the Virtual Campus that are 2- to 4-weeks in duration. There are middle and high school competition math camps as well as Math Beasts camps that review key topics coupled with fun explorations covering areas such as graph theory (Math Beasts Camp 6), cryptography (Math Beasts Camp 7-8), and topology (Math Beasts Camp 8-9)!

Be sure to mark your calendars for the following events:
[list][*]April 3rd (Webinar), 4pm PT/7:00pm ET, Learning with AoPS: Perspectives from a Parent, Math Camp Instructor, and University Professor
[*]April 8th (Math Jam), 4:30pm PT/7:30pm ET, 2025 MATHCOUNTS State Discussion
April 9th (Webinar), 4:00pm PT/7:00pm ET, Learn about Video-based Summer Camps at the Virtual Campus
[*]April 10th (Math Jam), 4:30pm PT/7:30pm ET, 2025 MathILy and MathILy-Er Math Jam: Multibackwards Numbers
[*]April 22nd (Webinar), 4:00pm PT/7:00pm ET, Competitive Programming at AoPS (USACO).[/list]
Our full course list for upcoming classes is below:
All classes run 7:30pm-8:45pm ET/4:30pm - 5:45pm PT unless otherwise noted.

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0 replies
jlacosta
Apr 2, 2025
0 replies
k i Peer-to-Peer Programs Forum
jwelsh   157
N Dec 11, 2023 by cw357
Many of our AoPS Community members share their knowledge with their peers in a variety of ways, ranging from creating mock contests to creating real contests to writing handouts to hosting sessions as part of our partnership with schoolhouse.world.

To facilitate students in these efforts, we have created a new Peer-to-Peer Programs forum. With the creation of this forum, we are starting a new process for those of you who want to advertise your efforts. These advertisements and ensuing discussions have been cluttering up some of the forums that were meant for other purposes, so we’re gathering these topics in one place. This also allows students to find new peer-to-peer learning opportunities without having to poke around all the other forums.

To announce your program, or to invite others to work with you on it, here’s what to do:

1) Post a new topic in the Peer-to-Peer Programs forum. This will be the discussion thread for your program.

2) Post a single brief post in this thread that links the discussion thread of your program in the Peer-to-Peer Programs forum.

Please note that we’ll move or delete any future advertisement posts that are outside the Peer-to-Peer Programs forum, as well as any posts in this topic that are not brief announcements of new opportunities. In particular, this topic should not be used to discuss specific programs; those discussions should occur in topics in the Peer-to-Peer Programs forum.

Your post in this thread should have what you're sharing (class, session, tutoring, handout, math or coding game/other program) and a link to the thread in the Peer-to-Peer Programs forum, which should have more information (like where to find what you're sharing).
157 replies
jwelsh
Mar 15, 2021
cw357
Dec 11, 2023
k i C&P posting recs by mods
v_Enhance   0
Jun 12, 2020
The purpose of this post is to lay out a few suggestions about what kind of posts work well for the C&P forum. Except in a few cases these are mostly meant to be "suggestions based on historical trends" rather than firm hard rules; we may eventually replace this with an actual list of firm rules but that requires admin approval :) That said, if you post something in the "discouraged" category, you should not be totally surprised if it gets locked; they are discouraged exactly because past experience shows they tend to go badly.
-----------------------------
1. Program discussion: Allowed
If you have questions about specific camps or programs (e.g. which classes are good at X camp?), these questions fit well here. Many camps/programs have specific sub-forums too but we understand a lot of them are not active.
-----------------------------
2. Results discussion: Allowed
You can make threads about e.g. how you did on contests (including AMC), though on AMC day when there is a lot of discussion. Moderators and administrators may do a lot of thread-merging / forum-wrangling to keep things in one place.
-----------------------------
3. Reposting solutions or questions to past AMC/AIME/USAMO problems: Allowed
This forum contains a post for nearly every problem from AMC8, AMC10, AMC12, AIME, USAJMO, USAMO (and these links give you an index of all these posts). It is always permitted to post a full solution to any problem in its own thread (linked above), regardless of how old the problem is, and even if this solution is similar to one that has already been posted. We encourage this type of posting because it is helpful for the user to explain their solution in full to an audience, and for future users who want to see multiple approaches to a problem or even just the frequency distribution of common approaches. We do ask for some explanation; if you just post "the answer is (B); ez" then you are not adding anything useful.

You are also encouraged to post questions about a specific problem in the specific thread for that problem, or about previous user's solutions. It's almost always better to use the existing thread than to start a new one, to keep all the discussion in one place easily searchable for future visitors.
-----------------------------
4. Advice posts: Allowed, but read below first
You can use this forum to ask for advice about how to prepare for math competitions in general. But you should be aware that this question has been asked many many times. Before making a post, you are encouraged to look at the following:
[list]
[*] Stop looking for the right training: A generic post about advice that keeps getting stickied :)
[*] There is an enormous list of links on the Wiki of books / problems / etc for all levels.
[/list]
When you do post, we really encourage you to be as specific as possible in your question. Tell us about your background, what you've tried already, etc.

Actually, the absolute best way to get a helpful response is to take a few examples of problems that you tried to solve but couldn't, and explain what you tried on them / why you couldn't solve them. Here is a great example of a specific question.
-----------------------------
5. Publicity: use P2P forum instead
See https://artofproblemsolving.com/community/c5h2489297_peertopeer_programs_forum.
Some exceptions have been allowed in the past, but these require approval from administrators. (I am not totally sure what the criteria is. I am not an administrator.)
-----------------------------
6. Mock contests: use Mock Contests forum instead
Mock contests should be posted in the dedicated forum instead:
https://artofproblemsolving.com/community/c594864_aops_mock_contests
-----------------------------
7. AMC procedural questions: suggest to contact the AMC HQ instead
If you have a question like "how do I submit a change of venue form for the AIME" or "why is my name not on the qualifiers list even though I have a 300 index", you would be better off calling or emailing the AMC program to ask, they are the ones who can help you :)
-----------------------------
8. Discussion of random math problems: suggest to use MSM/HSM/HSO instead
If you are discussing a specific math problem that isn't from the AMC/AIME/USAMO, it's better to post these in Middle School Math, High School Math, High School Olympiads instead.
-----------------------------
9. Politics: suggest to use Round Table instead
There are important conversations to be had about things like gender diversity in math contests, etc., for sure. However, from experience we think that C&P is historically not a good place to have these conversations, as they go off the rails very quickly. We encourage you to use the Round Table instead, where it is much more clear that all posts need to be serious.
-----------------------------
10. MAA complaints: discouraged
We don't want to pretend that the MAA is perfect or that we agree with everything they do. However, we chose to discourage this sort of behavior because in practice most of the comments we see are not useful and some are frankly offensive.
[list] [*] If you just want to blow off steam, do it on your blog instead.
[*] When you have criticism, it should be reasoned, well-thought and constructive. What we mean by this is, for example, when the AOIME was announced, there was great outrage about potential cheating. Well, do you really think that this is something the organizers didn't think about too? Simply posting that "people will cheat and steal my USAMOO qualification, the MAA are idiots!" is not helpful as it is not bringing any new information to the table.
[*] Even if you do have reasoned, well-thought, constructive criticism, we think it is actually better to email it the MAA instead, rather than post it here. Experience shows that even polite, well-meaning suggestions posted in C&P are often derailed by less mature users who insist on complaining about everything.
[/list]
-----------------------------
11. Memes and joke posts: discouraged
It's fine to make jokes or lighthearted posts every so often. But it should be done with discretion. Ideally, jokes should be done within a longer post that has other content. For example, in my response to one user's question about olympiad combinatorics, I used a silly picture of Sogiita Gunha, but it was done within a context of a much longer post where it was meant to actually make a point.

On the other hand, there are many threads which consist largely of posts whose only content is an attached meme with the word "MAA" in it. When done in excess like this, the jokes reflect poorly on the community, so we explicitly discourage them.
-----------------------------
12. Questions that no one can answer: discouraged
Examples of this: "will MIT ask for AOIME scores?", "what will the AIME 2021 cutoffs be (asked in 2020)", etc. Basically, if you ask a question on this forum, it's better if the question is something that a user can plausibly answer :)
-----------------------------
13. Blind speculation: discouraged
Along these lines, if you do see a question that you don't have an answer to, we discourage "blindly guessing" as it leads to spreading of baseless rumors. For example, if you see some user posting "why are there fewer qualifiers than usual this year?", you should not reply "the MAA must have been worried about online cheating so they took fewer people!!". Was sich überhaupt sagen lässt, lässt sich klar sagen; und wovon man nicht reden kann, darüber muss man schweigen.
-----------------------------
14. Discussion of cheating: strongly discouraged
If you have evidence or reasonable suspicion of cheating, please report this to your Competition Manager or to the AMC HQ; these forums cannot help you.
Otherwise, please avoid public discussion of cheating. That is: no discussion of methods of cheating, no speculation about how cheating affects cutoffs, and so on --- it is not helpful to anyone, and it creates a sour atmosphere. A longer explanation is given in Seriously, please stop discussing how to cheat.
-----------------------------
15. Cutoff jokes: never allowed
Whenever the cutoffs for any major contest are released, it is very obvious when they are official. In the past, this has been achieved by the numbers being posted on the official AMC website (here) or through a post from the AMCDirector account.

You must never post fake cutoffs, even as a joke. You should also refrain from posting cutoffs that you've heard of via email, etc., because it is better to wait for the obvious official announcement. A longer explanation is given in A Treatise on Cutoff Trolling.
-----------------------------
16. Meanness: never allowed
Being mean is worse than being immature and unproductive. If another user does something which you think is inappropriate, use the Report button to bring the post to moderator attention, or if you really must reply, do so in a way that is tactful and constructive rather than inflammatory.
-----------------------------

Finally, we remind you all to sit back and enjoy the problems. :D

-----------------------------
(EDIT 2024-09-13: AoPS has asked to me to add the following item.)

Advertising paid program or service: never allowed

Per the AoPS Terms of Service (rule 5h), general advertisements are not allowed.

While we do allow advertisements of official contests (at the MAA and MATHCOUNTS level) and those run by college students with at least one successful year, any and all advertisements of a paid service or program is not allowed and will be deleted.
0 replies
v_Enhance
Jun 12, 2020
0 replies
k i Stop looking for the "right" training
v_Enhance   50
N Oct 16, 2017 by blawho12
Source: Contest advice
EDIT 2019-02-01: https://blog.evanchen.cc/2019/01/31/math-contest-platitudes-v3/ is the updated version of this.

EDIT 2021-06-09: see also https://web.evanchen.cc/faq-contest.html.

Original 2013 post
50 replies
v_Enhance
Feb 15, 2013
blawho12
Oct 16, 2017
inquequality
ngocthi0101   9
N 30 minutes ago by sqing
let $a,b,c > 0$ prove that
$\frac{a}{b} + \sqrt {\frac{b}{c}}  + \sqrt[3]{{\frac{c}{a}}} > \frac{5}{2}$
9 replies
ngocthi0101
Sep 26, 2014
sqing
30 minutes ago
square root problem that involves geometry
kjhgyuio   1
N 34 minutes ago by kjhgyuio
If x is a nonnegative real number , find the minimum value of √x^2+4 + √x^2 -24x +153

1 reply
kjhgyuio
35 minutes ago
kjhgyuio
34 minutes ago
Assisted perpendicular chasing
sarjinius   5
N an hour ago by hukilau17
Source: Philippine Mathematical Olympiad 2025 P7
In acute triangle $ABC$ with circumcenter $O$ and orthocenter $H$, let $D$ be an arbitrary point on the circumcircle of triangle $ABC$ such that $D$ does not lie on line $OB$ and that line $OD$ is not parallel to line $BC$. Let $E$ be the point on the circumcircle of triangle $ABC$ such that $DE$ is perpendicular to $BC$, and let $F$ be the point on line $AC$ such that $FA = FE$. Let $P$ and $R$ be the points on the circumcircle of triangle $ABC$ such that $PE$ is a diameter, and $BH$ and $DR$ are parallel. Let $M$ be the midpoint of $DH$.
(a) Show that $AP$ and $BR$ are perpendicular.
(b) Show that $FM$ and $BM$ are perpendicular.
5 replies
sarjinius
Mar 9, 2025
hukilau17
an hour ago
Tangent.
steven_zhang123   2
N an hour ago by AshAuktober
Source: China TST 2001 Quiz 6 P1
In \( \triangle ABC \) with \( AB > BC \), a tangent to the circumcircle of \( \triangle ABC \) at point \( B \) intersects the extension of \( AC \) at point \( D \). \( E \) is the midpoint of \( BD \), and \( AE \) intersects the circumcircle of \( \triangle ABC \) at \( F \). Prove that \( \angle CBF = \angle BDF \).
2 replies
steven_zhang123
Mar 23, 2025
AshAuktober
an hour ago
Looks Like Mount Inequality Erupted :(
jasonhu4   161
N 6 hours ago by Marcus_Zhang
Source: 2017 USAMO #6
Find the minimum possible value of \[\frac{a}{b^3+4}+\frac{b}{c^3+4}+\frac{c}{d^3+4}+\frac{d}{a^3+4}\]given that $a$, $b$, $c$, $d$ are nonnegative real numbers such that $a+b+c+d=4$.

Proposed by Titu Andreescu
161 replies
jasonhu4
Apr 20, 2017
Marcus_Zhang
6 hours ago
usamOOK geometry
KevinYang2.71   93
N Yesterday at 8:07 PM by Bonime
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 $XY$.
93 replies
KevinYang2.71
Mar 21, 2025
Bonime
Yesterday at 8:07 PM
2025 Math and AI 4 Girls Competition: Win Up To $1,000!!!
audio-on   21
N Yesterday at 6:26 PM by mkwhe
Join the 2025 Math and AI 4 Girls Competition for a chance to win up to $1,000!

Hey Everyone, I'm pleased to announce the dates for the 2025 MA4G Competition are set!
Applications will open on March 22nd, 2025, and they will close on April 26th, 2025 (@ 11:59pm PST).

Applicants will have one month to fill out an application with prizes for the top 50 contestants & cash prizes for the top 20 contestants (including $1,000 for the winner!). More details below!

Eligibility:
The competition is free to enter, and open to middle school female students living in the US (5th-8th grade).
Award recipients are selected based on their aptitude, activities and aspirations in STEM.

Event dates:
Applications will open on March 22nd, 2025, and they will close on April 26th, 2025 (by 11:59pm PST)
Winners will be announced on June 28, 2025 during an online award ceremony.

Application requirements:
Complete a 12 question problem set on math and computer science/AI related topics
Write 2 short essays

Prizes:
1st place: $1,000 Cash prize
2nd place: $500 Cash prize
3rd place: $300 Cash prize
4th-10th: $100 Cash prize each
11th-20th: $50 Cash prize each
Top 50 contestants: Over $50 worth of gadgets and stationary


Many thanks to our current and past sponsors and partners: Hudson River Trading, MATHCOUNTS, Hewlett Packard Enterprise, Automation Anywhere, JP Morgan Chase, D.E. Shaw, and AI4ALL.

Math and AI 4 Girls is a nonprofit organization aiming to encourage young girls to develop an interest in math and AI by taking part in STEM competitions and activities at an early age. The organization will be hosting an inaugural Math and AI 4 Girls competition to identify talent and encourage long-term planning of academic and career goals in STEM.

Contact:
mathandAI4girls@yahoo.com

For more information on the competition:
https://www.mathandai4girls.org/math-and-ai-4-girls-competition

More information on how to register will be posted on the website. If you have any questions, please ask here!


21 replies
audio-on
Jan 26, 2025
mkwhe
Yesterday at 6:26 PM
Question about USAMO, self esteem, and college
xHypotenuse   25
N Yesterday at 6:04 PM by anticodon
Hello everyone. I know this question may sound ridiculous/neagtive but I really want to know how the rest of the community thinks on this issue. Please excuse this yap session and feel free to ignore this post if it doesn't make sense, I don't think I really have a sane mind these days and something has gotten into my head.

I want your advice on what I should do in this situation. It has been my dream to make usamo since ~second semester of 9th grade and I started grinding from that time on. Last year, I qualified for the aime and got a 5. This year I really wanted to qualify for the olympiad and studied really hard. I spent my entire summer working on counting and probability, the subject I suck at the most. And yet, on amc 12, I fumbled hard. I usually mocked ~120-130s on amc 10s but on amc 12 this year, I got really mediocre scores ~100. So I had no chance of making usamo.

So during winter of 2024-2025 I kinda gave up on aime studying and I was like "hey, if I can't get into usamo, maybe ill qualify for usapho." Since I was pretty good at physics at that time. So I spended my winter hard grinding for f=ma and guess what? The test had stupid and ridiculous questions and I only got an 11. What really sucks is that even with the stupid amount of cheaters in f=ma, if I changed all of my "D" guesses to "C," then I would have qualified. Since I solved 10 actually and guessed the rest. Absolutely unfair that only 1 of my guesses were correct.

And also since I didn't study for aime, I ended up being super rusty and so I only got a 7. Solved 9 tho. (I usually can consistently solve 10+ on aimes).

And now here's my senior year and ofc I want to apply to a prestigious college. But it feels stupid that I don't have any usamo or usapho titles like the people I know do. I think I will have good essays primarily due to a varied amount of life experiences but like, I don't feel like I will contribute much to the college without being some prestigious olympiad qualifier. So this led to me having a self esteem issue.

This also led me to the question: should I study one last year so that I can get into usamo in my senior year, or is there no point? Since like, colleges don't care about whatever the hell you do in your senior year, and also, it seems just 'weird' to be grinding math contests while the rest of the people from my school are playing around, etc. So this time around I've really been having an internal crisis between my self esteem (since getting into usamo will raise my self esteem a lot) and college/senior choices.

I know this may seem like a dumb question to some and you are free to completely ignore the post. That's fine. I just really want advice for what I should do in this situation and it would really help bring my life quality up

Thanks,
hypotenuse
25 replies
xHypotenuse
Apr 3, 2025
anticodon
Yesterday at 6:04 PM
Catch those negatives
cappucher   40
N Yesterday at 4:19 PM by Apple_maths60
Source: 2024 AMC 10A P11
How many ordered pairs of integers $(m, n)$ satisfy $\sqrt{n^2 - 49} = m$?

$
\textbf{(A) }1 \qquad
\textbf{(B) }2 \qquad
\textbf{(C) }3 \qquad
\textbf{(D) }4 \qquad
\textbf{(E) } \text{Infinitely many} \qquad
$
40 replies
cappucher
Nov 7, 2024
Apple_maths60
Yesterday at 4:19 PM
People who know that 23 divides 2024:
Marcus_Zhang   28
N Yesterday at 3:55 PM by Apple_maths60
Source: 2024 AMC 10A #5/2024 AMC 12A #4
What is the least value of $n$ such that $n!$ is a multiple of $2024$?

$
\textbf{(A) }11 \qquad
\textbf{(B) }21 \qquad
\textbf{(C) }22 \qquad
\textbf{(D) }23 \qquad
\textbf{(E) }253 \qquad
$
28 replies
Marcus_Zhang
Nov 7, 2024
Apple_maths60
Yesterday at 3:55 PM
sum to 2024
pog   25
N Yesterday at 3:50 PM by Apple_maths60
Source: 2024 AMC 10A #4 / 2024 AMC 12A #3
The number $2024$ is written as the sum of not necessarily distinct two-digit numbers. What is the least number of two-digit numbers needed to write this sum?

$\textbf{(A) }20\qquad\textbf{(B) }21\qquad\textbf{(C) }22\qquad\textbf{(D) }23\qquad\textbf{(E) }24$
25 replies
pog
Nov 7, 2024
Apple_maths60
Yesterday at 3:50 PM
its giving SAT math
centslordm   47
N Yesterday at 3:42 PM by Apple_maths60
Source: 2024 AMC 10A #2, AMC 12A #2
A model used to estimate the time it will take to hike to the top of the mountain on a trail is of the form $T = aL + bG,$ where $a$ and $b$ are constants, $T$ is the time in minutes, $L$ is the length of the trail in miles, and $G$ is the altitude gain in feet. The model estimates that it will take $69$ minutes to hike to the top if a trail is $1.5$ miles long and ascends $800$ feet, as well as if a trail is $1.2$ miles long and ascends $1100$ feet. How many minutes does the model estimate it will take to hike to the top if the trail is $4.2$ miles long and ascends $4000$ feet?

$\textbf{(A) } 240 \qquad \textbf{(B) } 246 \qquad \textbf{(C) } 252 \qquad \textbf{(D) } 258 \qquad \textbf{(E) } 264$
47 replies
centslordm
Nov 7, 2024
Apple_maths60
Yesterday at 3:42 PM
prime spam
fruitmonster97   28
N Yesterday at 3:28 PM by Apple_maths60
Source: 2024 AMC 10A #3
What is the sum of the digits of the smallest prime that can be written as a sum of $5$ distinct primes?

$\textbf{(A) }5\qquad\textbf{(B) }7\qquad\textbf{(C) }9\qquad\textbf{(D) }10\qquad\textbf{(E) }11$
28 replies
fruitmonster97
Nov 7, 2024
Apple_maths60
Yesterday at 3:28 PM
five digit multiplication?
fruitmonster97   48
N Yesterday at 3:15 PM by Apple_maths60
Source: 2024 AMC 10A #1/AMC 12A #1
What is the value of $9901\cdot101-99\cdot10101?$

$\textbf{(A) }2\qquad\textbf{(B) }20\qquad\textbf{(C) }21\qquad\textbf{(D) }200\qquad\textbf{(E) }2020$
48 replies
fruitmonster97
Nov 7, 2024
Apple_maths60
Yesterday at 3:15 PM
Angles in parallelogram
juckter   7
N Feb 7, 2022 by JAnatolGT_00
Source: Mexican Mathematical Olympiad 2013 Problem 2
Let $ABCD$ be a parallelogram with the angle at $A$ obtuse. Let $P$ be a point on segment $BD$. The circle with center $P$ passing through $A$ cuts line $AD$ at $A$ and $Y$ and cuts line $AB$ at $A$ and $X$. Line $AP$ intersects $BC$ at $Q$ and $CD$ at $R$. Prove $\angle XPY = \angle XQY + \angle XRY$.
7 replies
juckter
Dec 1, 2013
JAnatolGT_00
Feb 7, 2022
Angles in parallelogram
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Source: Mexican Mathematical Olympiad 2013 Problem 2
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juckter
322 posts
#1 • 1 Y
Y by Adventure10
Let $ABCD$ be a parallelogram with the angle at $A$ obtuse. Let $P$ be a point on segment $BD$. The circle with center $P$ passing through $A$ cuts line $AD$ at $A$ and $Y$ and cuts line $AB$ at $A$ and $X$. Line $AP$ intersects $BC$ at $Q$ and $CD$ at $R$. Prove $\angle XPY = \angle XQY + \angle XRY$.
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Arab
612 posts
#2 • 2 Y
Y by Adventure10, Mango247
$\frac{AP}{PQ}=\frac{DP}{BP}=\frac{PR}{AP}$,and hence $PA^2=PX^2=PY^2=PQ\cdot PR$.Therefore we obtain
$\angle XPY=\angle PXR+\angle PYR+\angle XRY=\angle PQX+\angle PQY+\angle XRY=\angle XQY+$ $\angle XRY$,as desired.
$Q.E.D.$
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Vo Duc Dien
341 posts
#3 • 1 Y
Y by Adventure10
Anyone sees that the circle in the problem is an Apollonian circle? The problem is more beautiful than the simple answer we have seen, and because of the circle being Apollonian, the result still holds true even if $Y$ is the intersection of the circle with side $CD$ ($Y$ is one of the intersection points that is closer to $D$.)
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jayme
9775 posts
#4 • 2 Y
Y by Adventure10, Mango247
Dear Mathlinkers,
nice observation....
Sincerely
Jean-Louis
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Vo Duc Dien
341 posts
#5 • 2 Y
Y by Adventure10, Mango247
That creates a new problem: Prove that it's an Apollonian circle! ... :-)
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Arab
612 posts
#6 • 2 Y
Y by Adventure10, Mango247
I think my solution shows clearly it is an Apollonian circle defined by $Q,R$ by proving that $PY^2=PQ\cdot PR$,which implies that $P$ is the center of the Apollonian circle.
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Mahdi_Mashayekhi
689 posts
#7
Y by
AP/PQ = PD/PB = PR/PA so PA^2 = PR.PQ so XRQ is tangent to XP and with same approach we can prove YRQ is tangent to YP.
∠XQY = ∠XQP + ∠YQP = ∠RXP + ∠RYP so ∠XQY + ∠XRY = ∠XQP + ∠YQP + ∠XRY = ∠XPY.
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JAnatolGT_00
559 posts
#8
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Since $\frac{|AP|}{|PQ|}=\frac{|DP|}{|PB|}=\frac{|RP|}{|PA|},$ inversion wrt $(P)$ swaps $Q,R,$ and therefore $$\angle XQY+\angle XRY=\angle PXR+\angle PYR+\angle XRY=\angle XPY.$$
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