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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
May 1, 2025
May is an exciting month! National MATHCOUNTS is the second week of May in Washington D.C. and our Founder, Richard Rusczyk will be presenting a seminar, Preparing Strong Math Students for College and Careers, on May 11th.

Are you interested in working towards MATHCOUNTS and don’t know where to start? We have you covered! If you have taken Prealgebra, then you are ready for MATHCOUNTS/AMC 8 Basics. Already aiming for State or National MATHCOUNTS and harder AMC 8 problems? Then our MATHCOUNTS/AMC 8 Advanced course is for you.

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[*]May 21st, 4:00pm PT/7:00pm ET, Mathcamp 2025 Qualifying Quiz Part 2 Math Jam, Problems 5 and 6, Canada/USA Mathcamp staff will discuss solutions to Problems 5 and 6 of the 2025 Mathcamp Qualifying Quiz![/list]
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0 replies
jlacosta
May 1, 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
B.Stat & B.Math 2022 - Q8
integrated_JRC   6
N 27 minutes ago by Titeer_Bhar
Source: Indian Statistical Institute (ISI) - B.Stat & B.Math Entrance 2022
Find the minimum value of $$\big|\sin x+\cos x+\tan x+\cot x+\sec x+\operatorname{cosec}x\big|$$for real numbers $x$ not multiple of $\frac{\pi}{2}$.
6 replies
integrated_JRC
May 8, 2022
Titeer_Bhar
27 minutes ago
AD=BE implies ABC right
v_Enhance   117
N 30 minutes ago by cj13609517288
Source: European Girl's MO 2013, Problem 1
The side $BC$ of the triangle $ABC$ is extended beyond $C$ to $D$ so that $CD = BC$. The side $CA$ is extended beyond $A$ to $E$ so that $AE = 2CA$. Prove that, if $AD=BE$, then the triangle $ABC$ is right-angled.
117 replies
v_Enhance
Apr 10, 2013
cj13609517288
30 minutes ago
IMO Genre Predictions
ohiorizzler1434   64
N 39 minutes ago by ariopro1387
Everybody, with IMO upcoming, what are you predictions for the problem genres?


Personally I predict: predict
64 replies
ohiorizzler1434
May 3, 2025
ariopro1387
39 minutes ago
3-var inequality
sqing   2
N an hour ago by pooh123
Source: Own
Let $ a,b\geq  0 ,a^3-ab+b^3=1  $. Prove that
$$  \frac{1}{2}\geq     \frac{a}{a^2+3 }+ \frac{b}{b^2+3}   \geq  \frac{1}{4}$$$$  \frac{1}{2}\geq     \frac{a}{a^3+3 }+ \frac{b}{b^3+3}   \geq  \frac{1}{4}$$$$  \frac{1}{2}\geq \frac{a}{a^2+ab+2}+ \frac{b}{b^2+ ab+2}  \geq  \frac{1}{3}$$$$  \frac{1}{2}\geq \frac{a}{a^3+ab+2}+ \frac{b}{b^3+ ab+2}  \geq  \frac{1}{3}$$Let $ a,b\geq  0 ,a^3+ab+b^3=3  $. Prove that
$$  \frac{1}{2}\geq     \frac{a}{a^2+3 }+ \frac{b}{b^2+3}   \geq  \frac{1}{4}(\frac{1}{\sqrt[3]{3}}+\sqrt[3]{3}-1)$$$$  \frac{1}{2}\geq     \frac{a}{a^3+3 }+ \frac{b}{b^3+3}   \geq  \frac{1}{2\sqrt[3]{9}}$$$$  \frac{1}{2}\geq \frac{a}{a^2+ab+2}+ \frac{b}{b^2+ ab+2}  \geq  \frac{4\sqrt[3]{3}+3\sqrt[3]{9}-6}{17}$$$$  \frac{1}{2}\geq \frac{a}{a^3+ab+2}+ \frac{b}{b^3+ ab+2}  \geq  \frac{\sqrt[3]{3}}{5}$$
2 replies
sqing
Today at 4:32 AM
pooh123
an hour ago
3-var inequality
sqing   1
N an hour ago by sqing
Source: Own
Let $ a,b>0 $ and $\frac{1}{a^2+3}+ \frac{1}{b^2+ 3} \leq \frac{1}{2} . $ Prove that
$$a^2+ab+b^2\geq 3$$$$a^2-ab+b^2 \geq 1 $$Let $ a,b>0 $ and $\frac{1}{a^3+3}+ \frac{1}{b^3+ 3}\leq \frac{1}{2} . $ Prove that
$$a^3+ab+b^3 \geq 3$$$$ a^3-ab+b^3\geq 1 $$
1 reply
sqing
an hour ago
sqing
an hour ago
Iranians playing with cards module a prime number.
Ryan-asadi   2
N an hour ago by AshAuktober
Source: Iranian Team Selection Test - P2
.........
2 replies
Ryan-asadi
3 hours ago
AshAuktober
an hour ago
Coloring plane in black
Ryan-asadi   1
N 2 hours ago by AshAuktober
Source: Iran Team Selection Test - P3
..........
1 reply
Ryan-asadi
3 hours ago
AshAuktober
2 hours ago
An analytic sequence
Ryan-asadi   1
N 2 hours ago by AshAuktober
Source: Iran Team Selection Test - P1
..........
1 reply
1 viewing
Ryan-asadi
4 hours ago
AshAuktober
2 hours ago
Geometry
gggzul   6
N 2 hours ago by Captainscrubz
In trapezoid $ABCD$ segments $AB$ and $CD$ are parallel. Angle bisectors of $\angle A$ and $\angle C$ meet at $P$. Angle bisectors of $\angle B$ and $\angle D$ meet at $Q$. Prove that $ABPQ$ is cyclic
6 replies
gggzul
Yesterday at 8:22 AM
Captainscrubz
2 hours ago
Need help on this simple looking problem
TheGreatEuler   0
2 hours ago
Show that 1+2+3+4....n divides 1^k+2^k+3^k....n^k when k is odd. Is this possible to prove without using congruence modulo or binomial coefficients?
0 replies
TheGreatEuler
2 hours ago
0 replies
Geometry
Lukariman   5
N 2 hours ago by Lukariman
Given circle (O) and point P outside (O). From P draw tangents PA and PB to (O) with contact points A, B. On the opposite ray of ray BP, take point M. The circle circumscribing triangle APM intersects (O) at the second point D. Let H be the projection of B on AM. Prove that $\angle HDM$ = 2∠AMP.
5 replies
Lukariman
Yesterday at 12:43 PM
Lukariman
2 hours ago
inq , not two of them =0
win14   0
2 hours ago
Let a,b,c be non negative real numbers such that no two of them are simultaneously equal to 0
$$\frac{1}{a + b} + \frac{1}{b + c} + \frac{1}{c + a} \ge \frac{5}{2\sqrt{ab + bc + ca}}.$$
0 replies
win14
2 hours ago
0 replies
Number theory
MathsII-enjoy   5
N 3 hours ago by MathsII-enjoy
Prove that when $x^p+y^p$ | $(p^2-1)^n$ with $x,y$ are positive integers and $p$ is prime ($p>3$), we get: $x=y$
5 replies
MathsII-enjoy
Monday at 3:22 PM
MathsII-enjoy
3 hours ago
Combinatorics
P162008   4
N 4 hours ago by cazanova19921
Let $m,n \in \mathbb{N}.$ Let $[n]$ denote the set of natural numbers less than or equal to $n.$

Let $f(m,n) = \sum_{(x_1,x_2,x_3, \cdots, x_m) \in [n]^{m}} \frac{x_1}{x_1 + x_2 + x_3 + \cdots + x_m} \binom{n}{x_1} \binom{n}{x_2} \binom{n}{x_3} \cdots \binom{n}{x_m} 2^{\left(\sum_{i=1}^{m} x_i\right)}$

Compute the sum of the digits of $f(4,4).$
4 replies
P162008
Today at 5:38 AM
cazanova19921
4 hours ago
a MID(point) geo problem
sketchydealer05   63
N Apr 11, 2025 by Avron
Source: EGMO 2023/2
We are given an acute triangle $ABC$. Let $D$ be the point on its circumcircle such that $AD$ is a diameter. Suppose that points $K$ and $L$ lie on segments $AB$ and $AC$, respectively, and that $DK$ and $DL$ are tangent to circle $AKL$.
Show that line $KL$ passes through the orthocenter of triangle $ABC$.
63 replies
sketchydealer05
Apr 16, 2023
Avron
Apr 11, 2025
a MID(point) geo problem
G H J
G H BBookmark kLocked kLocked NReply
Source: EGMO 2023/2
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allaith.sh
26 posts
#59
Y by
let $M$ the midpoint of $KL$.
claim $1$: $M \in AH$
proof: $AD$ is $A-symmedian$ of $AKL$
then $\angle CAH= \angle KAD = \angle MAC$.$ _\blacksquare$
claim $2$: $LCDM$ cyclic
proof: $\angle LCD= \angle ACD = \angle LMD. _\blacksquare$
claim $3$: $M \in CH$
proof: $180^{\circ}-\angle 2A = \angle KDL$$ \implies \angle LDM = 90^{\circ}-\angle A = \angle LCM = \angle ACM ._\blacksquare$
by claim $1$ and $3$ we have $ M = CH \cap AH =H$.
and we are done.
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lelouchvigeo
181 posts
#60
Y by
Too Easy :-D
This problem is basically this
https://artofproblemsolving.com/community/q2h535000p29584904
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TestX01
341 posts
#61
Y by
We do $\sqrt {bc}$ inversion centred at $A$. :P

Let $H$ be the orthocentre of $\triangle ABC$. Let $E,F,G$ be the altitudes of $A,B,C$ onto $BC,AC,AB$ respectively. The inversion sends $E$ to $D$, and sends $F$ to the point on $AB$ such that $ACF'$ is right, and similar for $G'$. Now the orthocentre is the intersection of $(AFG)$ with $AE$, so $H'$ is the intersection of $F'G'$ with $AD$.

It is trivial to see that $D$ is the orthocenter of $\triangle AF'G'$, so $AD$ is simply the altitude of $A$ onto $F'G'$. Now see where $K,L$ are sent to. $K'L'$ is tangent to $AK'E$ and $AL'E$ at $K',L'$. This is equivalent to $E$ being the $A-HM$ point of $AK'L'$.

Now since $AE$ and $AD$ are isogonal, the orthocentre of $AK'L'$ is the intersection of the perpendicular to $AE$ through $E$ with $AD$, which is precisely the intersection of $BC$ with $AD$. We want to show that $K, H,L$ are collinear or that $A,K',H',L'$ are concyclic.

Let $P$ be the intersection of $BC$ with $AD$, since $P$ is the orthocentre of $AK'L'$, $L'P$ is parallel to $CD$, and $K'P$ is parallel to $BD$. We claim that $L'K'$ is parallel to $F'G'$, but this is trivial since: $\frac{AL'}{AF'}=\frac{AP}{AD}=\frac{AK'}{AG'}$.

Now we want the dilation at $A$ with power $\frac{AP}{AD}$ to take the circumcircle of $\triangle AF'G'$ to the "circumcircle" of $AK'H'L'$, hence it suffices to show that $H'$ is sent to a point on the circumcircle of $AF'G'$ in this dilation.

Let $AD$ intersect $(AF'G')$ at a point $Q$. It suffices to prove that $\frac{AP}{AD}=\frac{AH'}{AQ}$ or just $AP\times AQ=AD\times AH'$. Now, $AP\times AQ=AC\times AG'=AD\times AH'$ by repeated Power of a Point, so we are done, as $AK'H'L'$ is cyclic and $KHL$ are collinear.
Attachments:
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shendrew7
795 posts
#62
Y by
Define $M$ as the midpoint of $KL$ to form cyclic quadrilaterals $BKMD$ and $CLMD$. Proving $M=H$ boils down to showing $BDCM$ is a parallelogram, which is true as
\[\angle CMB = \angle DKB + \angle CLD = \angle ALK + \angle AKL = 180-\angle A = \angle BDC\]\[\angle MBD = \angle MKD = \angle DLM = \angle DCM. \quad \blacksquare\]
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InterLoop
280 posts
#63
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xoink
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L13832
268 posts
#64
Y by
Let $M$ be the midpoint of $KL$. The motivation behind considering the midpoint is that $AD$ is $A-$symmedian of $\triangle AKL$ so $AM$ will be the isogonal conjugate and by angle chasing we will have $\measuredangle CAM=\measuredangle CAH$)
To Prove: $M$ is the orthocenter of $\triangle ABC$.
Claim: $\odot(DBKM)$
Proof: $$\measuredangle DBK = \measuredangle DMK = 90^\circ$$So we have
$$\measuredangle DBM = \measuredangle DKM = \measuredangle BAC = \measuredangle HBD$$and therefore $BM \perp AC$. Similarly $CM \perp AB$, so $M$ is the orthocenter of $ABC$.
This post has been edited 3 times. Last edited by L13832, Oct 11, 2024, 7:08 AM
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Markas
105 posts
#65
Y by
From the tangents we have that $\angle DLK = \angle DKL = \angle CAK = \alpha$. We will prove that $H \equiv M$, where M is the midpoint of LK, then $H \in LK$. We know that M is the midpoint of LK $\Rightarrow$ $\angle LMD = \angle DMK = 90^{\circ}$ $\Rightarrow$ $\angle LDM = \angle KDM = 90 - \alpha$. Now $\angle LCD + \angle LMD = 90 + 90 = 180^{\circ}$ $\Rightarrow$ LMDC is cyclic. Similarly since $\angle KBD + \angle KMD = 90 + 90 = 180^{\circ}$, MKBD is cyclic. From the cyclic quadrilaterals it follows that $\angle LCM = \angle LDM = 90 - \alpha$ and $\angle KBM = \angle KDM = 90 - \alpha$ $\Rightarrow$ $\angle ACM = 90 - \alpha$ and $\angle ABM = 90 - \alpha$ $\Rightarrow$ $M \in C_h$ and $M \in B_h$, where $B_h$ and $C_h$ are the heights from B and C respectively $\Rightarrow$ $M \equiv H$ $\Rightarrow$ H is the midpoint of LK $\Rightarrow$ $H \in LK$ and we are ready.
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abeot
125 posts
#66 • 1 Y
Y by centslordm
Very beautiful problem!
[asy]
import geometry;
unitsize(4cm);
pair A = dir(110); pair B = dir(220); pair C = dir(320);
pair H = A+B+C; pair G = dir(250);
pair M = (B+C)/2; pair O = 0; pair Y = reflect( perpendicular(O, line(G,M))) * G; pair D = -A; pair T = intersectionpoint(perpendicular(B,line(O,B)), perpendicular(C,line(O,C)));
pair K = intersectionpoint(perpendicular(H,line(H,D)), line(A,B)); pair L = intersectionpoint(perpendicular(H,line(H,D)), line(A,C));
draw(A--B--C--cycle); draw(circle(A,K,L), dashed); draw(D--K); draw(D--L); draw(circle(A,B,C)); draw(Y--T); draw(A--G); draw(Y--B); draw(Y--C); draw(Y--G); draw(A--D); draw(K--L); draw(T--B); draw(T--C);
draw(A--H--D--cycle, red+1.1); draw(Y--M--T--cycle, red+1.1);
filldraw(A--H--D--cycle, invisible, red+1.1);
filldraw(Y--M--T--cycle, invisible, red+1.1);
filldraw(A--K--L--cycle, invisible, blue+1.1);
filldraw(Y--B--C--cycle, invisible, blue+1.1);
dot("$A$", A, dir(110)); dot("$D$", D, dir(290)); dot("$B$", B, dir(210)); dot("$C$", C, dir(330)); dot("$H$", H, dir(0));
dot("$G$", G, dir(250)); dot("$M$", M, dir(135)); dot("$Y$", Y, dir(60)); dot("$T$", T, dir(270)); dot("$O$", O, dir(120)); dot("$K$", K, dir(180)); dot("$L$", L, dir(30));  [/asy]
Notice that $AD$ is the $A$-symmedian in $\triangle AKL$. Thus \[ \frac{AK}{AL} = \frac{\sin KAD}{\sin LAD} = \frac{\cos C}{\cos B} = \frac{HC}{HB} \]where $H$ is the orthocenter of $\triangle ABC$. Define $G$ as the reflection of $H$ over $BC$ and $M$ as the midpoint of $BC$. Let $GM$ intersect $(ABC)$ again at $Y$.

Claim: We have $\triangle YBC \sim \triangle AKL$.
Proof. Then $YD$ and $YG$ are isogonal wrt $\angle BYC$ so $YBDC$ is harmonic, implying \[ \frac{YB}{BC} = \frac{DB}{DC} = \frac{GC}{GB} = \frac{HC}{HB} \]hence $\triangle YBC \sim \triangle AKL$. $\blacksquare$
Now, let $T$ be the pole of $BC$ wrt $(ABC)$. Then the ratio of the spiral similarity sending $\triangle YBC$ to $\triangle AKL$ is $\frac{YT}{AD}$ (both are the segments on the $A$-symmedian joining the vertex to the opposite pole). Moreover, $YDT$ is collinear by the definition of the symmedian.

Claim: We have $\triangle MYT \sim \triangle HAD$.
Proof. Note $\angle MYT = \angle GYD = \angle HAD$. If $O$ is the center of $(ABC)$, note that $OM \cdot OT = OY^2$ by the definition of pole. Thus $\triangle OMY \sim \triangle OYT$, and so \[ \frac{YM}{YT} = \frac{OM}{OY} = \frac{OM}{OB} = \cos A = \frac{AH}{AD} \]thus $\triangle MYT \sim \triangle HAD$. $\blacksquare$

But then the spiral similarity sending $YT$ to $AD$ (which is the same one that sends $\triangle YBC$ to $\triangle AKL$) also sends $YM$ to $AH$. By similarity then $H$ is the midpoint of $KL$. $\square$
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zaidova
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#67
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Main part is just constructing the midpoint of $KL$(Let's say it $M$) . Then from angle chasing $H \equiv M$ (orthocenter lemma helps for that problem)
This post has been edited 2 times. Last edited by zaidova, Dec 25, 2024, 8:44 PM
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Davud29_09
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#68
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We note that AH perpendicular to BC and H is on KL.We want to prove H is the orthocenter.We get with AD is symmedian AH is median.We get DH perpendicular to KL.Then BKHD,LHDC is cyclic.We get with easy angle-chasing BH is perpendicular to AC.We are done.
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D4N13LCarpenter
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#69 • 1 Y
Y by Vahe_Arsenyan
Let $M$ be the midpoint of $KL$. Notice that $\angle DHK=\angle DBK=90^\circ$ so $BMDK$ is cyclic. Analogously we get $HLCD$ cyclic, which implies $$\angle HBD=\angle HCD = \alpha.$$However, we also have $\angle HBD=\angle HCD= \alpha$ so indeed $M=H$.
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Avron
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#70
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Let $H'$ be the midpoint of $KL$. Notice that
$$\angle{KH'D}=\angle{LH'D}=\angle{KBD}=\angle{LCD}=90$$so $BDH'K$, $DCLH'$ are both cyclic and
$$\angle{KBH'}=\angle{KDH'}=90-\angle{HKD}=90-\angle{A}$$same for $\angle{LCH'}=\angle{A}$ so $H'=H$ and we're done.
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ErTeeEs06
64 posts
#71 • 1 Y
Y by Funcshun840
Let circles $(BDK)$ and $(CDL)$ intersect at $S\neq D$. Then $\angle DSK=180^\circ-\angle DBA=\angle DCA=180^\circ-\angle DSL$ so $K, S, L$ are collinear. Now $$\angle DBS=\angle DKS=\angle DLS=\angle DCS$$and $$\angle BSC=\angle BKD+\angle CLD=\angle KLA+\angle LKA=180-\angle BAC=\angle BDC$$This implies $BDCS$ is a parallelogram so $S$ is orthocenter of $\triangle ABC$ and we're done.
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Tony_stark0094
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#72
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let $H$ be the midpoint of line $EF$ now we wish to prove that $H$ is indeed the orthocentre of $\Delta ABC$
note that $AG$ is the symmedian of $\Delta AEF$ $\implies \angle EAH =\angle FAG =\angle CBD = 90-\angle B  $
$\implies AH$ is perpendicular to $BC$
also $\angle DHE=\angle DBA=\angle DBE=90$ $\implies E,B,D,H$ are concyclic points
$\angle EDH= \frac {\angle EDF}{2}= \frac{180-2\angle A}{2} =90-\angle A$
$\implies \angle ABH=\angle EBH= \angle EDH =90 - \angle A$
$\implies BH$ is perpendicular to $AC$ hence $H $ is the orthocenter with respect to $\Delta ABC$
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Avron
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#73
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Let $H$ be the intersection of the altitude from $A$ and $KL$, we'll prove that it is the orthocenter. It is well known that the orthocenter and the circumcenter are isogonal conjugates so $AH$ is isogonal to $AD$ but $AD$ is the symedian so $H$ is the midpoint of $KL$. This implies $\angle KHD=\angle DHL=\angle DCL=\angle DBK = 90$ so $KHDB$ and $LHCD$ are cyclic. Let $\angle A = \alpha$, now
\[
\angle HBD=\angle HKD = \alpha, \angle HCD=\angle HLD=\alpha
\]also $\angle BDC=180-\alpha$ and $\angle BHC=360-(\angle  HCD + \angle HBD + \angle BDC)=180-\alpha = \angle BDC$ so $HBDC$ is a parallelogram and $H$ is the reflection of $D$ w.r.t the midpoint of $BC$ as desired.
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