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k a March Highlights and 2025 AoPS Online Class Information
jlacosta   0
Mar 2, 2025
March is the month for State MATHCOUNTS competitions! Kudos to everyone who participated in their local chapter competitions and best of luck to all going to State! Join us on March 11th for a Math Jam devoted to our favorite Chapter competition problems! Are you interested in training for MATHCOUNTS? Be sure to check out our AMC 8/MATHCOUNTS Basics and Advanced courses.

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Do you have plans this summer? There are so many options to fit your schedule and goals whether attending a summer camp or taking online classes, it can be a great break from the routine of the school year. Check out our summer courses at AoPS Online, or if you want a math or language arts class that doesn’t have homework, but is an enriching summer experience, our AoPS Virtual Campus summer camps may be just the ticket! We are expanding our locations for our AoPS Academies across the country with 15 locations so far and new campuses opening in Saratoga CA, Johns Creek GA, and the Upper West Side NY. Check out this page for summer camp information.

Be sure to mark your calendars for the following events:
[list][*]March 5th (Wednesday), 4:30pm PT/7:30pm ET, HCSSiM Math Jam 2025. Amber Verser, Assistant Director of the Hampshire College Summer Studies in Mathematics, will host an information session about HCSSiM, a summer program for high school students.
[*]March 6th (Thursday), 4:00pm PT/7:00pm ET, Free Webinar on Math Competitions from elementary through high school. Join us for an enlightening session that demystifies the world of math competitions and helps you make informed decisions about your contest journey.
[*]March 11th (Tuesday), 4:30pm PT/7:30pm ET, 2025 MATHCOUNTS Chapter Discussion MATH JAM. AoPS instructors will discuss some of their favorite problems from the MATHCOUNTS Chapter Competition. All are welcome!
[*]March 13th (Thursday), 4:00pm PT/7:00pm ET, Free Webinar about Summer Camps at the Virtual Campus. Transform your summer into an unforgettable learning adventure! From elementary through high school, we offer dynamic summer camps featuring topics in mathematics, language arts, and competition preparation - all designed to fit your schedule and ignite your passion for learning.[/list]
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0 replies
jlacosta
Mar 2, 2025
0 replies
k i Adding contests to the Contest Collections
dcouchman   1
N Apr 5, 2023 by v_Enhance
Want to help AoPS remain a valuable Olympiad resource? Help us add contests to AoPS's Contest Collections.

Find instructions and a list of contests to add here: https://artofproblemsolving.com/community/c40244h1064480_contests_to_add
1 reply
dcouchman
Sep 9, 2019
v_Enhance
Apr 5, 2023
k i Zero tolerance
ZetaX   49
N May 4, 2019 by NoDealsHere
Source: Use your common sense! (enough is enough)
Some users don't want to learn, some other simply ignore advises.
But please follow the following guideline:


To make it short: ALWAYS USE YOUR COMMON SENSE IF POSTING!
If you don't have common sense, don't post.


More specifically:

For new threads:


a) Good, meaningful title:
The title has to say what the problem is about in best way possible.
If that title occured already, it's definitely bad. And contest names aren't good either.
That's in fact a requirement for being able to search old problems.

Examples:
Bad titles:
- "Hard"/"Medium"/"Easy" (if you find it so cool how hard/easy it is, tell it in the post and use a title that tells us the problem)
- "Number Theory" (hey guy, guess why this forum's named that way¿ and is it the only such problem on earth¿)
- "Fibonacci" (there are millions of Fibonacci problems out there, all posted and named the same...)
- "Chinese TST 2003" (does this say anything about the problem¿)
Good titles:
- "On divisors of a³+2b³+4c³-6abc"
- "Number of solutions to x²+y²=6z²"
- "Fibonacci numbers are never squares"


b) Use search function:
Before posting a "new" problem spend at least two, better five, minutes to look if this problem was posted before. If it was, don't repost it. If you have anything important to say on topic, post it in one of the older threads.
If the thread is locked cause of this, use search function.

Update (by Amir Hossein). The best way to search for two keywords in AoPS is to input
[code]+"first keyword" +"second keyword"[/code]
so that any post containing both strings "first word" and "second form".


c) Good problem statement:
Some recent really bad post was:
[quote]$lim_{n\to 1}^{+\infty}\frac{1}{n}-lnn$[/quote]
It contains no question and no answer.
If you do this, too, you are on the best way to get your thread deleted. Write everything clearly, define where your variables come from (and define the "natural" numbers if used). Additionally read your post at least twice before submitting. After you sent it, read it again and use the Edit-Button if necessary to correct errors.


For answers to already existing threads:


d) Of any interest and with content:
Don't post things that are more trivial than completely obvious. For example, if the question is to solve $x^{3}+y^{3}=z^{3}$, do not answer with "$x=y=z=0$ is a solution" only. Either you post any kind of proof or at least something unexpected (like "$x=1337, y=481, z=42$ is the smallest solution). Someone that does not see that $x=y=z=0$ is a solution of the above without your post is completely wrong here, this is an IMO-level forum.
Similar, posting "I have solved this problem" but not posting anything else is not welcome; it even looks that you just want to show off what a genius you are.

e) Well written and checked answers:
Like c) for new threads, check your solutions at least twice for mistakes. And after sending, read it again and use the Edit-Button if necessary to correct errors.



To repeat it: ALWAYS USE YOUR COMMON SENSE IF POSTING!


Everything definitely out of range of common sense will be locked or deleted (exept for new users having less than about 42 posts, they are newbies and need/get some time to learn).

The above rules will be applied from next monday (5. march of 2007).
Feel free to discuss on this here.
49 replies
ZetaX
Feb 27, 2007
NoDealsHere
May 4, 2019
Problem 4
den_thewhitelion   3
N 2 minutes ago by DensSv
Source: Second Romanian JBMO TST 2016
We have a 4x4 board.All 1x1 squares are white.A move is changing colours of all squares of a 1x3 rectangle from black to white and from white to black.It is possible to make all the 1x1 squares black after several moves?
3 replies
den_thewhitelion
Jun 15, 2016
DensSv
2 minutes ago
Find the period
Anto0110   2
N 6 minutes ago by YaoAOPS
Let $a_1, a_2, ..., a_k, ...$ be a sequence that consists of an initial block of $p$ positive distinct integers that then repeat periodically. This means that $\{a_1, a_2, \dots, a_p\}$ are $p$ distinct positive integers and $a_{n+p}=a_n$ for every positive integer $n$. The terms of the sequence are not known and the goal is to find the period $p$. To do this, at each move it possible to reveal the value of a term of the sequence at your choice.
If $p$ is one of the first $k$ prime numbers, find for which values of $k$ there exist a strategy that allows to find $p$ revealing at most $8$ terms of the sequence.
2 replies
Anto0110
Yesterday at 7:37 PM
YaoAOPS
6 minutes ago
Interesting inequality
sqing   3
N 8 minutes ago by sqing
Source: Own
Let $ a,b,c\geq 2  . $ Prove that
$$(a^2-1)(b-1)(c^2-1) -\frac{9}{4}abc\geq -9$$$$(a^2-1)(b-1)(c^2-1) -\frac{11}{5}abc\geq -\frac{43}{5}$$$$(a^2-1)(b-1)(c^2-1) -2abc\geq -7$$$$(a-1)(b^2-1)(c-1) -\frac{3}{4}abc\geq -3$$$$(a-1)(b^2-1)(c-1) -\frac{3}{5}abc\geq -\frac{9}{5}$$$$(a-1)(b^2-1)(c-1) -\frac{1}{2}abc\geq -1$$
3 replies
sqing
2 hours ago
sqing
8 minutes ago
Very interesting inequality
sqing   0
17 minutes ago
Source: Own
Let $ a,b,c\geq 2  . $ Prove that
$$(a-1)(b^2-2)(c^3-3)-  \frac{5}{2}abc\geq -10$$$$(a-\frac{3}{2})(b^2-2)(c^3-3)-  \frac{5}{2}abc\geq -15$$$$(a-\frac{3}{2})(b^2-\frac{3}{2})(c^3-3)-  \frac{25}{8}abc\geq - \frac{155}{8}$$$$(a-\frac{3}{2})(b^2-\frac{3}{2})(c^3-3)- 3abc\geq - \frac{363}{20}$$$$(a-\frac{3}{2})(b^2-\frac{3}{2})(c^3-\frac{5}{2})- \frac{55}{16}abc\geq - \frac{341}{16}$$
0 replies
sqing
17 minutes ago
0 replies
inequality
senku23   3
N 27 minutes ago by SunnyEvan
Let x,y,z in R+ prove that 8(x^3+y^3+z^3)2≥9(x^2+yz)(y^2+zx)(z^2+xy).
3 replies
senku23
3 hours ago
SunnyEvan
27 minutes ago
Cool Number Theory
Fermat_Fanatic108   1
N an hour ago by Fermat_Fanatic108
For an integer with 5 digits $n=abcde$ (where $a, b, c, d, e$ are the digits and $a\neq 0$) we define the \textit{permutation sum} as the value $$bcdea+cdeab+deabc+eabcd$$For example the permutation sum of 20253 is $$02532+25320+53202+32025=113079$$Let $m$ and $n$ be two fivedigit integers with the same permutation sum.
Prove that $m=n$.
1 reply
Fermat_Fanatic108
an hour ago
Fermat_Fanatic108
an hour ago
ratio chasing inside a triangle, segment trisecting
parmenides51   10
N an hour ago by sangsidhya
Source: CRMO 2012 Region 2 p5
Let $ABC$ be a triangle. Let $D, E$ be a points on the segment $BC$ such that $BD =DE = EC$. Let $F$ be the mid-point of $AC$. Let $BF$ intersect $AD$ in $P$ and $AE$ in $Q$ respectively. Determine $BP:PQ$.
10 replies
parmenides51
Sep 30, 2018
sangsidhya
an hour ago
Geo: incircle, escircle, isotomic conjugate
XAN4   0
an hour ago
Source: Own
For $\triangle{ABC}$, Its incircle $\odot I$ and $A-$escircle $\odot I_A$ are tangent to $BC$ at $D$ and $E$ respectively. $AI$ intersects line $BC$ at $J$. Line $AD$ intersects $\odot I$ at $F$, and line $AE$ intersects $\odot I_A$ at $G$. Line $FG$ intersects $BC$ at $H$. Prove that $BJ=CH$.
0 replies
XAN4
an hour ago
0 replies
f(x)+f(x+y) \leq f(xy)+f(y)
augustin_p   7
N an hour ago by MuradSafarli
Source: Estonia TST 2022
Find all functions $f:\mathbb{R}\rightarrow\mathbb{R}$ that satisfy the following condition for any real numbers $x{}$ and $y$ $$f(x)+f(x+y) \leq f(xy)+f(y).$$
7 replies
augustin_p
Jul 12, 2023
MuradSafarli
an hour ago
Integer equation in 3 variables
Kimchiks926   2
N an hour ago by MuradSafarli
Source: Latvian TST for Baltic Way 2019 Problem 15
Determine all tuples of integers $(a,b,c)$ such that:
$$(a-b)^3(a+b)^2 = c^2 + 2(a-b) + 1$$
2 replies
Kimchiks926
May 29, 2020
MuradSafarli
an hour ago
Interesting inequality
sqing   1
N an hour ago by sqing
Source: Own
Let $ a,b,c\geq 2  . $ Prove that
$$(a^2-1)(b-\frac{3}{2})(c^2-1) - \frac{9}{4}abc\geq -15$$$$(a^2-1)(b-\frac{3}{2})(c^2-1) - 2abc\geq -\frac{73}{6}$$$$(a^2-1)(b-\frac{3}{2})(c^2-1) - abc\geq -\frac{7}{2}$$$$(a^2-2)(b-\frac{3}{2})(c^2-2) - abc\geq -6$$
1 reply
sqing
an hour ago
sqing
an hour ago
Hard problem involving circumcenter and concurrent lines
GeoMetrix   6
N an hour ago by bin_sherlo
Source: AQGO 2020 Problem 3
Let $\triangle{ABC}$ be a triangle with circumcenter $O$. Let $M,N$ be the midpoints of $\overline{AB}$ and $\overline{AC}$ respectively and let $T$ be the projection of $O$ on $\overline{MN}$. Let $D$ be the projection of $A$ on $\overline{BC}$. Let $\overline{TD}$ intersect $\odot(BOC)$ at points $U$ and $V$. Let $\odot(AUV)$ intersct $\overline{MN}$ at points $X,Y$. Let $\overline{AY}$ intersect $\odot(AMN)$ at $R$ and $\overline{AX}$ intersect $\odot(AMN)$ at $S$. Then show that $\overline{AO},\overline{RS},\overline{MN}$ are concurrent.

Proposed by GeoMetrix
6 replies
+1 w
GeoMetrix
Jun 20, 2020
bin_sherlo
an hour ago
Oi! These lines concur
Rg230403   17
N an hour ago by L13832
Source: LMAO 2021 P5, LMAOSL G3(simplified)
Let $I, O$ and $\Gamma$ respectively be the incentre, circumcentre and circumcircle of triangle $ABC$. Points $A_1, A_2$ are chosen on $\Gamma$, such that $AA_1 = AI = AA_2$, and point $A'$ is the foot of the altitude from $I$ to $A_1A_2$. If $B', C'$ are similarly defined, prove that lines $AA', BB'$ and $CC'$ concurr on $OI$.
Original Version from SL
Proposed by Mahavir Gandhi
17 replies
Rg230403
May 10, 2021
L13832
an hour ago
Differentiable functional
bakerbakura   2
N 2 hours ago by Gryphos
Find all differentiable functions $ f;\mathbb{R}\to\mathbb{R}$ such that, for all real numbers $ a,b,t$ with $ 0<t<1$, $ t^2f(a)+(1-t^2)f(b)\geq f(ta+(1-t)b)$
2 replies
bakerbakura
Jan 11, 2010
Gryphos
2 hours ago
IMO 2023 P2
799786   89
N Yesterday at 1:36 AM by kaede_Arcadia
Source: IMO 2023 P2
Let $ABC$ be an acute-angled triangle with $AB < AC$. Let $\Omega$ be the circumcircle of $ABC$. Let $S$ be the midpoint of the arc $CB$ of $\Omega$ containing $A$. The perpendicular from $A$ to $BC$ meets $BS$ at $D$ and meets $\Omega$ again at $E \neq A$. The line through $D$ parallel to $BC$ meets line $BE$ at $L$. Denote the circumcircle of triangle $BDL$ by $\omega$. Let $\omega$ meet $\Omega$ again at $P \neq B$. Prove that the line tangent to $\omega$ at $P$ meets line $BS$ on the internal angle bisector of $\angle BAC$.
89 replies
799786
Jul 8, 2023
kaede_Arcadia
Yesterday at 1:36 AM
IMO 2023 P2
G H J
G H BBookmark kLocked kLocked NReply
Source: IMO 2023 P2
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799786
1052 posts
#1 • 8 Y
Y by Quidditch, PRMOisTheHardestExam, LoloChen, KST2003, trinhquockhanh, Rounak_iitr, Funcshun840, cubres
Let $ABC$ be an acute-angled triangle with $AB < AC$. Let $\Omega$ be the circumcircle of $ABC$. Let $S$ be the midpoint of the arc $CB$ of $\Omega$ containing $A$. The perpendicular from $A$ to $BC$ meets $BS$ at $D$ and meets $\Omega$ again at $E \neq A$. The line through $D$ parallel to $BC$ meets line $BE$ at $L$. Denote the circumcircle of triangle $BDL$ by $\omega$. Let $\omega$ meet $\Omega$ again at $P \neq B$. Prove that the line tangent to $\omega$ at $P$ meets line $BS$ on the internal angle bisector of $\angle BAC$.
This post has been edited 5 times. Last edited by Amir Hossein, Aug 8, 2023, 5:43 PM
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khan.academy
633 posts
#3 • 1 Y
Y by LoloChen
Let $ABC$ be an acute-angled triangle with $AB < AC$. Let $\Omega$ be the circumcircle of $ABC$. Let $S$ be the midpoint of the arc $CB$ of $\Omega$ containing $A$. The perpendicular from $A$ to $BC$ meets $BS$ at $D$ and meets $\Omega$ again at $E \neq A$. The line through $D$ parallel to $BC$ meets line $BE$ at $L$. Denote the circumcircle of triangle $BDL$ by $\omega$. Let $\omega$ meet $\Omega$ again at $P \neq B$. Prove that the line tangent to $\omega$ at $P$ meets line $BS$ on the internal angle bisector of $\angle BAC$.

@above it is
This post has been edited 1 time. Last edited by khan.academy, Jul 8, 2023, 4:52 AM
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LoloChen
475 posts
#4 • 18 Y
Y by 799786, khan.academy, khina, David-Vieta, GeoKing, oolite, ike.chen, Before-the-snow-melted, Aryan-23, sabkx, Dhruv777, Gato_combinatorio, aidan0626, PennyLane_31, AlexCenteno2007, ehuseyinyigit, LLL2019, Sedro
Too easy for P2! :(
Sketch: construct antipode of A to get rid of L and find concyclics kills it.
https://cdn.artofproblemsolving.com/attachments/c/5/7b46be3b7184784a85e64fdf9db4642e647f9c.png
Construct midpoint of arc $BC$ not containing ${A}$ and $A-$ antipode to be $N$ and $A'$. Let $AN$ cut $SB$ at $Y$ and $DA$ at $X$. Let ${AE}$ cut $SA'$ at $T$.
$\angle L=\angle EBC=\angle BPD$ so $LPDB$ concyclic. Note that $A'S//AN$ and by Reim $PYXB$ concyclic. Since $\angle TEA'=90$ and $SE=SA'$, $A'S=TS$ so by homothety at ${D}$, $AY=YX$. Combining $\angle APX=90$, $PY=XY$, $\angle YPD= \angle YXD=\angle PBY$, so $PY$ is tangent to $\omega$.
This post has been edited 3 times. Last edited by LoloChen, Jul 8, 2023, 2:56 PM
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tastymath75025
3223 posts
#9 • 4 Y
Y by khina, centslordm, ehuseyinyigit, jkim0656
Let $A',S'$ be the antipodes to $A,S$ on $(ABC)$. Defining $X = AS' \cap BS$, we want to show $PX$ is tangent to $(BPD)$ or $XD\cdot XB = XP^2$.

First, note by Reim on $(LPDB), (PBEA')$ that since $LD||EA'$ we must have $P,D,A'$ are collinear. Next, note that $\angle DBA = \angle S'AE = \angle XAD$, so $XA$ is tangent to $(BAD)$ and thus $XD\cdot XB = XA^2$, so to finish we only need to show $XA=XP$. The locus of such $X$ is the $A$-midline of $\triangle AA'P$, so if we let $Y=AX\cap A'D$ we just need to show $X$ is the midpoint of $AY$.

But if we let $Q = AE \cap SA'$ then from angle-chasing $QAA'$ is isosceles, so $AS\perp A'S$ implies $S$ is the midpoint of $QA'$. Taking homothety centered at $D$ therefore shows $X$ is the midpoint of $AY$, as desired $\blacksquare$

Motivation: The motivation for this solution is to first eliminate $L$ using Reim to get a nicer characterization for $P$. After this, dealing with $XP$ and $(BPD)$ directly is still not nice, but we bypass this by noticing $XA$ is tangent to $(BAD)$.
This post has been edited 7 times. Last edited by tastymath75025, Jul 8, 2023, 6:17 AM
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trinhquockhanh
522 posts
#14 • 4 Y
Y by PRMOisTheHardestExam, GeoKing, Rounak_iitr, ehuseyinyigit
$\text{No more cyclic quadrilaterals}$
https://i.ibb.co/yfc8FBc/2023-IMO-P2.png
This post has been edited 2 times. Last edited by trinhquockhanh, Jul 9, 2023, 3:52 PM
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Aryan-23
558 posts
#15 • 1 Y
Y by PRMOisTheHardestExam
Let $A'$ denote the $A$ antipode. We have $\angle BPD = \angle EBC = \angle BCA' =\angle BPA'$, so $P,D,A'$ are collinear.

Let $AI$ hit $PA'$ at $H$ and $BD$ at $F$. Let $AE$ and $SA'$ intersect at $J$. Then Note that, $FH \equiv AI\parallel SA'$, so that $PFHB$ is cyclic by Reims. Now note that $JA'E$ is a right triangle with $SA'=SE$ (as $A'E \parallel BC$), so $SA'=SJ$. From homothety at $D$ we get $FA=FH$. Now as $\triangle APH$ is right, we have $FP = FH$. So we get $\angle FPA'= \angle FHP= \angle SA'P = \angle SBP = \angle DBP$.

So $FP$ is tangent to $\omega$, as desired. $\square$.
This post has been edited 1 time. Last edited by Aryan-23, Jul 8, 2023, 6:40 AM
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KST2003
173 posts
#17 • 2 Y
Y by PRMOisTheHardestExam, Rounak_iitr
Let $M$ be the midpoint of minor arc $BC$, and let $\overline{PM}$ intersect $\overline{AE}$ at $K$. Then $\overline{SM} \parallel \overline{DK}$, so $K$ lies on $\omega$ by Reim's. Now redefine $F$ to be the intersection of $\overline{AM}$ and $\overline{BS}$. We will show that $\overline{FP}$ is tangent to $\omega$.

Since $\angle KBE = \angle LDE = 90^\circ$, we have
\[ \measuredangle BFA = \measuredangle SFM = 90^\circ + \measuredangle BMA = 90^\circ + \measuredangle BEA = \measuredangle BKA, \]so $AFKB$ is cyclic. Moreover,
\[ \measuredangle KBF = \measuredangle KAF = \measuredangle EAM = \measuredangle SMA = \measuredangle SBA = \measuredangle FBA,\]so $FA = FK$. Finally, $\measuredangle APK = \measuredangle APM = \measuredangle ASM = 90^\circ - \measuredangle SMA = 90^\circ - \measuredangle KAF$, so $F$ is the circumcenter of $\triangle PAK$ and hence $FP^2 = FA^2 = FD \cdot FB$. Hence $\overline{FP}$ is tangent to $\omega$ as desired.
This post has been edited 3 times. Last edited by KST2003, Jul 8, 2023, 6:56 AM
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MathLover_ZJ
826 posts
#19 • 5 Y
Y by David-Vieta, PRMOisTheHardestExam, Amiralizakeri2007, Rounak_iitr, ehuseyinyigit
Without antipodes: Let $\omega$ meet $AE$ at $T\neq D$.
The circumcenter of $\triangle APT$ is $Q$.
Connect $PT,BT,TQ,AP$.
then we have $\angle BPD=\angle BLD=90^{\circ}-\angle BED=\angle APB-90^{\circ},$
so $AP \perp PD$.
hence $\angle QPT=90^{\circ}-\angle PAT=\angle PDA=\angle PBT,$which means $PQ$ is tangent to $\omega$.
by $\angle AQT=360^{\circ}-2\angle APT=180^{\circ}-2\angle DBT=180^{\circ}-\angle ABT$ we know that $A,Q,T,B$ is concyclic.
since $AQ=QT\Rightarrow Q$ lies on $BS$.
we just need to prove $Q$ lies on the bisector.
$\Leftrightarrow \angle QAS=90^{\circ},$
which is obvious since $\angle AQB=\angle ATB,\angle TBE=90^{\circ}$.
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a22886
924 posts
#20 • 5 Y
Y by LoloChen, Aryan-23, David-Vieta, PRMOisTheHardestExam, Alfombra12
We consider the inversion $\Phi$ wrt $D$ that preserves $\Omega$.

Under $\Phi$ we have $A\leftrightarrow E$ and $B\leftrightarrow S$. It's natural to find$$\Phi(L)=\Phi(BE)\cap\Phi(DL)=\odot ADS\cap DL$$which indicates $\angle AS~\Phi(L)=\angle ADL=90^{\circ}$, i.e., $\Phi(\omega)=\operatorname{line}S\Phi(L)=SA'$ where $A'$ is the antipodal point of $A$.
From that, $\Phi(P)=\Phi(\omega)\cap\Phi(\Omega)=A'$, so $PDA'$ collinear.

Let the antipodal point of $S$ be $T$. $\angle APD=\angle SBT=90^{\circ}\Longrightarrow Q:=AP\cap BT\in\omega$
But also $AS=ET\Longrightarrow \overset{\LARGE \frown}{AP}+\overset{\LARGE \frown}{ET}=\overset{\LARGE \frown}{PS}\Longrightarrow R:=AE\cap PT\in\omega$
Now we apply Pascal's theorem to $(PPQBDR)$ to get the desired result.
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Parsia--
73 posts
#21
Y by
Let $A'$ be the antipode of $A$. We have $\angle BPD = \angle EBC = \angle BCA' =\angle BPA'$, so $P,D,A'$ are collinear.
Let $AE$ intersect $SA'$ at $I$ and let $AF$ intersect $PA'$ at $H$.
Since $SE=SA'$ and $\triangle IEA'$ is a right triangle, we get that $SA' = SE = SA$. Note that $AH \equiv AF || SA'$ so by Thales we get that $\frac{AF}{IS} = \frac{DF}{DS} = \frac{HF}{SA'}$ and so $AF = HF$. Note that $\triangle APH$ is a right triangle, so $PF = AF = HF$. Now note that $\angle DAF = \angle FAA' = \angle AA'S = \angle ABS = \angle ABF$ so $\triangle ADF \sim \triangle ABF$ and so $FP^2 = AF^2 = FD.FB$.
So $FP$ is tangent to $\omega$.
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giannis2006
45 posts
#22 • 1 Y
Y by Assassino9931
Let $N$ be the midpoint of the other arc $BC$ and $K= AD \cap PN$
Then $\angle BDK=\angle BDE =90-\angle DBC=90-\angle SBC=\angle BSN=\angle A/2=\angle BAN=\angle BPN=\angle BPK$ and so $K$ lies on $\omega$. Let now $H= AN \cap BS$. We need to show that $HP$ is tangent to $\omega$. It is obvious that $AE$, $SN$ are parallel and so $\angle DAH=\angle DAN=\angle ANS=\angle ABS=\angle ABD$, so $AH$ is tangent to $ABD$, i.e. $AH^2=HB*HD$, so it remains to prove $AH=HP$. Now we have from the cyclic quadrilaterals and the parallel lines: $\angle AKB=\angle DKB=$ $=180-\angle DLB$ $=180-\angle EBC$ $=180-\angle EAC$ $=180-\angle (90-C)$ $=90+\angle C$ $=90+\angle ANB$ $=\angle HBN+\angle HNB$ $=\angle AHB$ and hence $BKHA$ is cyclic, which gives that $\angle AKH=\angle ABH=\angle ABD=\angle DAH=\angle KAH$ and so $HA=HK$. Finally: $\angle AHK=\angle AHB+\angle BHK=\angle HBN+\angle HNB+\angle BAK=90+\angle C+\angle BAK=90+\angle C+90-\angle B=2\angle C+\angle A=2(\angle C+\angle A/2)=2(180-(\angle B+\angle A/2))=2(180-\angle ABN)=2(180-\angle APN)=
2(180-\angle APK)$ , from where we conclude that $H$ is the circumcenter of $APK$ and $AH=HP$, as needed.
This post has been edited 3 times. Last edited by giannis2006, Jul 8, 2023, 7:50 AM
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ericxyzhu
49 posts
#23 • 3 Y
Y by nervy, oolite, Funcshun840
This problem is proposed by Tiago Mourão, Nuno Arala, Portugal.
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juckter
322 posts
#24 • 3 Y
Y by PRMOisTheHardestExam, math90, UI_MathZ_25
Mediocre solution because I'm washed up. Let $T$ be the midpoint of arc $BC$ of $\Omega$ not containing $T$, let $R$ be the second intersection of $\omega$ with $AE$, and let $X = AT \cap BS$.

[asy]
import geometry;
unitsize(5cm);
defaultpen(fontsize(10pt));

point A = dir(120);
point B = dir(210);
point C = dir(330);
point I = incenter(A, B, C);
point O = circumcenter(A, B, C);
point T = circumcenter(I, B, C);
point S = 2 * O - T;
line l = bisector(S, T);
point E = 2 * (projection(l) * A) - A;
point D = extension(A, E, B, S);
point L = intersectionpoint(line(E, B), parallel(D, line(B,C)));
point R = 2 * circumcenter(B, D, L) - L;
point P = extension(L, S, T, R);
point X = extension(A, T, B, S);

draw(A--B--C--cycle, lightgray);
draw(S--L, gray);
draw(P--T, dashed);
draw(L--E--A);
draw(T--S);
draw(A--T, lightgray);
draw(B--S);
draw(A--P, gray);
draw(A--B--R--X--cycle, gray);
draw(circumcircle(A, B, C), blue);
draw(circumcircle(D, B, L), red);

dot(A);
dot(B);
dot(C);
dot(S);
dot(T);
dot(E);
dot(D);
dot(L);
dot(P);
dot(R);
dot(X);
label("$A$", A, N);
label("$B$", B, SW);
label("$C$", C, SE);
label("$D$", D, W);
label("$E$", E, (0,-1));
label("$L$", L, W);
label("$X$", X, (1, 0));
label("$R$", R, (1, 0));
label("$S$", S, N);
label("$T$", T, (0,-1));
label("$P$", P, N);
[/asy]

Claim 1. $L$, $P$ and $S$ are collinear.

Proof. Notice that the tangent to $\Omega$ at $S$ is parallel to $BC$ which is parallel to $DL$. Since $S, D, B$ are collinear, the converse of Reim's Theorem gives the result.

Claim 2. $T$, $R$ and $P$ are collinear.

Proof. Since $ST$ is a diameter of $\Omega$ we have $\angle TPS = 90^{\circ}$. On the other hand

\[\angle RPS = 180^{\circ} -  \angle RPL = 180^{\circ} - \angle RDL = 180^{\circ} - 90^{\circ}\]
And this $\angle RPS = \angle TPS$, giving the desired collinearity.

To finish, angle chase to find $\angle ABX = \angle XBR = \frac{\angle B - \angle C}{2} = \angle RAX$, so $AXRB$ is cyclic with $XA = XR$ and

\[\angle XRA = \angle XBA = \angle RBX = \angle RBD\]
And therefore $XR$ is tangent to $\omega$. It now suffices to show that $XR = XP$. By angle chase we have $\measuredangle RXA = 2\measuredangle RPA$, and since $XA = XR$ this implies that $X$ is the circumcenter of $\triangle APR$, finishing the problem.
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MarkBcc168
1593 posts
#25 • 2 Y
Y by PRMOisTheHardestExam, GeoKing
[asy]
size(9cm);
import olympiad;
import geometry;
pair A = dir(119);
pair A_prime = -A;
pair E = reflect((0,0), dir(90)) * A_prime;
pair S = dir(90);
pair D = 0.6*E + 0.4*A;
pair B = 2 * foot((0,0), D, S) - S;
pair P = 2 * foot((0,0), A_prime, D) - A_prime;
pair Q = reflect((0,0), (P+A_prime)/2) * S;
pair M = -S;
pair X = extension(E,Q,S,D);
pair Y = extension(P,Q,A,M);
pair L = extension(D+E-A_prime, D, B,E);
fill(X--M--Q--cycle, paleblue);
fill(A--P--S--cycle, paleblue);
draw(unitcircle, linewidth(1));
draw(S--X, red+linewidth(1.2));
draw(P--A_prime, linewidth(0.8));
draw(circle(B,D,P), linewidth(0.8));
draw(P--Q, linewidth(0.8));
draw(X--M--Q--cycle, blue+linewidth(0.8));
draw(A--P--S--cycle, blue+linewidth(0.8));
draw(A--M, linewidth(0.8));
draw(A--E--A_prime, gray+linewidth(0.7));
draw(S--M, gray+linewidth(0.7));
draw(D--L, gray+linewidth(0.7));
draw(M--A_prime);
dot("$A$", A, dir(118));
dot("$A'$", A_prime, dir(-62));
dot("$E$", E, dir(-84));
dot("$S$", S, dir(89));
dot("$D$", D, dir(16));
dot("$B$", B, 1.5*dir(-4));
dot("$P$", P, dir(133));
dot("$Q$", Q, dir(17));
dot("$M$", M, dir(-82));
dot("$X$", X, dir(-139));
dot("$Y$", Y, dir(143));
dot("$L$", L, dir(165));
[/asy]
Let $A'$ be the antipode of $A$ in $\Omega$. Moreover, let the tangent to $\omega$ at $P$ meet $\Omega$ again at $Q$. Let $M$ be the other midpoint of arc $BC$. Let $X=QE\cap MA'$, and $Y=PQ\cap AM$. We finish the problem in three steps.
  • Since $\angle BPD = \angle BLD = 90^\circ - \angle C = \angle BPA'$, we get that $P,D,A'$ are collinear.
  • Pascal on $QPA'MAE$ gives $PQ\cap AM=Y$, $PA'\cap AE = D$, and $A'M\cap EQ = X$ are collinear.
  • We have $\triangle QMX$ and $\triangle PAS$ are homothetic center $Y$. This is just angle chasing. To spell out details, we have
    • Reim on $PPQ$ and $BDS$ gives $SQ\parallel PD$, implying that $A'Q=SP$, or $\angle PAS=\angle QMX$,
    • $EM=AS$, so $\angle APS = MQX$, and
    • clearly, $MX\parallel AS$.
    Therefore, $S,Y,X$ are collinear.
Hence, $S, Y, D, X$ are collinear. $B$ clearly lies on this line, done.
This post has been edited 4 times. Last edited by MarkBcc168, Jul 8, 2023, 1:12 PM
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SQTHUSH
154 posts
#26
Y by
Let $K$ be the mid-point of arc $CB$ which is not containing $A$,suppose $AK\cap BS=T$,$PK\cap AE=E$
Note that $\angle BDE=\dfrac{1}{2}\angle A=\angle BPK$,so $P,B,Z,D$are cyclic
Consider that $\angle ABD=\angle B+\dfrac{1}{2}\angle A-90^{\circ}=90^{\circ}-\dfrac{1}{2}\angle A-\angle C=\angle DBZ=\angle EAK$
Which means that $A,B,Z,T$ are cyclic,and $TA=TC$
Notice $\angle ZAT=\angle TBZ=\angle ABT$,so $TA^{2}=TB\cdot TD$
Since $2\angle APK=2\angle B+\angle A=360^{\circ}-\angle ATZ$
Hence $TA=TZ=TP$
So $TP^{2}=TB\cdot TD$
Which means that $TP$ is tangent to $\omega$
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