Stay ahead of learning milestones! Enroll in a class over the summer!

G
Topic
First Poster
Last Poster
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.

Introductory: Grades 5-10

Prealgebra 1 Self-Paced

Prealgebra 1
Sunday, Apr 13 - Aug 10
Tuesday, May 13 - Aug 26
Thursday, May 29 - Sep 11
Sunday, Jun 15 - Oct 12
Monday, Jun 30 - Oct 20
Wednesday, Jul 16 - Oct 29

Prealgebra 2 Self-Paced

Prealgebra 2
Sunday, Apr 13 - Aug 10
Wednesday, May 7 - Aug 20
Monday, Jun 2 - Sep 22
Sunday, Jun 29 - Oct 26
Friday, Jul 25 - Nov 21

Introduction to Algebra A Self-Paced

Introduction to Algebra A
Monday, Apr 7 - Jul 28
Sunday, May 11 - Sep 14 (1:00 - 2:30 pm ET/10:00 - 11:30 am PT)
Wednesday, May 14 - Aug 27
Friday, May 30 - Sep 26
Monday, Jun 2 - Sep 22
Sunday, Jun 15 - Oct 12
Thursday, Jun 26 - Oct 9
Tuesday, Jul 15 - Oct 28

Introduction to Counting & Probability Self-Paced

Introduction to Counting & Probability
Wednesday, Apr 16 - Jul 2
Thursday, May 15 - Jul 31
Sunday, Jun 1 - Aug 24
Thursday, Jun 12 - Aug 28
Wednesday, Jul 9 - Sep 24
Sunday, Jul 27 - Oct 19

Introduction to Number Theory
Thursday, Apr 17 - Jul 3
Friday, May 9 - Aug 1
Wednesday, May 21 - Aug 6
Monday, Jun 9 - Aug 25
Sunday, Jun 15 - Sep 14
Tuesday, Jul 15 - Sep 30

Introduction to Algebra B Self-Paced

Introduction to Algebra B
Wednesday, Apr 16 - Jul 30
Tuesday, May 6 - Aug 19
Wednesday, Jun 4 - Sep 17
Sunday, Jun 22 - Oct 19
Friday, Jul 18 - Nov 14

Introduction to Geometry
Wednesday, Apr 23 - Oct 1
Sunday, May 11 - Nov 9
Tuesday, May 20 - Oct 28
Monday, Jun 16 - Dec 8
Friday, Jun 20 - Jan 9
Sunday, Jun 29 - Jan 11
Monday, Jul 14 - Jan 19

Intermediate: Grades 8-12

Intermediate Algebra
Monday, Apr 21 - Oct 13
Sunday, Jun 1 - Nov 23
Tuesday, Jun 10 - Nov 18
Wednesday, Jun 25 - Dec 10
Sunday, Jul 13 - Jan 18
Thursday, Jul 24 - Jan 22

Intermediate Counting & Probability
Wednesday, May 21 - Sep 17
Sunday, Jun 22 - Nov 2

Intermediate Number Theory
Friday, Apr 11 - Jun 27
Sunday, Jun 1 - Aug 24
Wednesday, Jun 18 - Sep 3

Precalculus
Wednesday, Apr 9 - Sep 3
Friday, May 16 - Oct 24
Sunday, Jun 1 - Nov 9
Monday, Jun 30 - Dec 8

Advanced: Grades 9-12

Olympiad Geometry
Tuesday, Jun 10 - Aug 26

Calculus
Tuesday, May 27 - Nov 11
Wednesday, Jun 25 - Dec 17

Group Theory
Thursday, Jun 12 - Sep 11

Contest Preparation: Grades 6-12

MATHCOUNTS/AMC 8 Basics
Wednesday, Apr 16 - Jul 2
Friday, May 23 - Aug 15
Monday, Jun 2 - Aug 18
Thursday, Jun 12 - Aug 28
Sunday, Jun 22 - Sep 21
Tues & Thurs, Jul 8 - Aug 14 (meets twice a week!)

MATHCOUNTS/AMC 8 Advanced
Friday, Apr 11 - Jun 27
Sunday, May 11 - Aug 10
Tuesday, May 27 - Aug 12
Wednesday, Jun 11 - Aug 27
Sunday, Jun 22 - Sep 21
Tues & Thurs, Jul 8 - Aug 14 (meets twice a week!)

AMC 10 Problem Series
Friday, May 9 - Aug 1
Sunday, Jun 1 - Aug 24
Thursday, Jun 12 - Aug 28
Tuesday, Jun 17 - Sep 2
Sunday, Jun 22 - Sep 21 (1:00 - 2:30 pm ET/10:00 - 11:30 am PT)
Monday, Jun 23 - Sep 15
Tues & Thurs, Jul 8 - Aug 14 (meets twice a week!)

AMC 10 Final Fives
Sunday, May 11 - Jun 8
Tuesday, May 27 - Jun 17
Monday, Jun 30 - Jul 21

AMC 12 Problem Series
Tuesday, May 27 - Aug 12
Thursday, Jun 12 - Aug 28
Sunday, Jun 22 - Sep 21
Wednesday, Aug 6 - Oct 22

AMC 12 Final Fives
Sunday, May 18 - Jun 15

F=ma Problem Series
Wednesday, Jun 11 - Aug 27

WOOT Programs
Visit the pages linked for full schedule details for each of these programs!


MathWOOT Level 1
MathWOOT Level 2
ChemWOOT
CodeWOOT
PhysicsWOOT

Programming

Introduction to Programming with Python
Thursday, May 22 - Aug 7
Sunday, Jun 15 - Sep 14 (1:00 - 2:30 pm ET/10:00 - 11:30 am PT)
Tuesday, Jun 17 - Sep 2
Monday, Jun 30 - Sep 22

Intermediate Programming with Python
Sunday, Jun 1 - Aug 24
Monday, Jun 30 - Sep 22

USACO Bronze Problem Series
Tuesday, May 13 - Jul 29
Sunday, Jun 22 - Sep 1

Physics

Introduction to Physics
Wednesday, May 21 - Aug 6
Sunday, Jun 15 - Sep 14
Monday, Jun 23 - Sep 15

Physics 1: Mechanics
Thursday, May 22 - Oct 30
Monday, Jun 23 - Dec 15

Relativity
Sat & Sun, Apr 26 - Apr 27 (4:00 - 7:00 pm ET/1:00 - 4:00pm PT)
Mon, Tue, Wed & Thurs, Jun 23 - Jun 26 (meets every day of the week!)
0 replies
jlacosta
Apr 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
Hard functional equation
pablock   31
N 12 minutes ago by bin_sherlo
Source: Brazil National Olympiad 2019 #3
Let $\mathbb{R}_{>0}$ be the set of the positive real numbers. Find all functions $f:\mathbb{R}_{>0} \rightarrow \mathbb{R}_{>0}$ such that $$f(xy+f(x))=f(f(x)f(y))+x$$for all positive real numbers $x$ and $y$.
31 replies
1 viewing
pablock
Nov 14, 2019
bin_sherlo
12 minutes ago
A cyclic inequality
KhuongTrang   9
N 19 minutes ago by KhuongTrang
Source: own-CRUX
IMAGE
Link
9 replies
+1 w
KhuongTrang
Apr 2, 2025
KhuongTrang
19 minutes ago
NEPAL TST DAY-2 PROBLEM 1
Tony_stark0094   8
N 28 minutes ago by Thapakazi


Let the sequence $\{a_n\}_{n \geq 1}$ be defined by
\[
a_1 = 1, \quad a_{n+1} = a_n + \frac{1}{\sqrt[2024]{a_n}} \quad \text{for } n \geq 1, \, n \in \mathbb{N}
\]Prove that
\[
a_n^{2025} >n^{2024}
\]for all positive integers $n \geq 2$.
8 replies
Tony_stark0094
5 hours ago
Thapakazi
28 minutes ago
Inspired by 2006 Romania
sqing   4
N 33 minutes ago by SunnyEvan
Source: Own
Let $a,b,c>0$ and $ abc= \frac{1}{8} .$ Prove that
$$   \frac{a+b}{c+1}+\frac{b+c}{a+1}+\frac{c+a}{b+1}-a-b-c  \geq \frac{1}{2}$$
4 replies
sqing
2 hours ago
SunnyEvan
33 minutes ago
Isosceles Triangle Geo
oVlad   1
N 40 minutes ago by Primeniyazidayi
Source: Romania Junior TST 2025 Day 1 P2
Consider the isosceles triangle $ABC$ with $\angle A>90^\circ$ and the circle $\omega$ of radius $AC$ centered at $A.$ Let $M$ be the midpoint of $AC.$ The line $BM$ intersects $\omega$ a second time at $D.$ Let $E$ be a point on $\omega$ such that $BE\perp AC.$ Let $N$ be the intersection of $DE$ and $AC.$ Prove that $AN=2\cdot AB.$
1 reply
oVlad
4 hours ago
Primeniyazidayi
40 minutes ago
Colouring numbers
kitun   3
N an hour ago by anudeep
What is the least number required to colour the integers $1, 2,.....,2^{n}-1$ such that for any set of consecutive integers taken from the given set of integers, there will always be a colour colouring exactly one of them? That is, for all integers $i, j$ such that $1<=i<=j<=2^{n}-1$, there will be a colour coloring exactly one integer from the set $i, i+1,.... , j-1, j$.
3 replies
+1 w
kitun
Nov 15, 2021
anudeep
an hour ago
Inequality while on a trip
giangtruong13   0
an hour ago
Source: Trip
I find this inequality while i was on a trip, it was pretty fun and i have some new experience:
Let $a,b,c \geq -2$ such that: $a^2+b^2+c^2 \leq 8$. Find the maximum: $$A= \sum_{cyc} \frac{1}{16+a^3}$$
0 replies
giangtruong13
an hour ago
0 replies
Nepal TST 2025 Day 1 Problem 2
Bata325   1
N an hour ago by ThatApollo777
Source: Nepal TST 2025, problem 1
Find all integers $n$ such that if
\[
1 = d_1 < d_2 < \cdots < d_{k-1} < d_k = n
\]are the divisors of $n$, then the sequence
\[
d_2 - d_1,\, d_3 - d_2,\, \ldots,\, d_k - d_{k-1}
\]forms a permutation of an arithmetic progression.(Kritesh Dhakal,Nepal)
1 reply
Bata325
Yesterday at 1:26 PM
ThatApollo777
an hour ago
Z[x] on set of integers, bounding closure measure of set
jasperE3   3
N an hour ago by Bigtaitus
Source: VJIMC 2013 1.3
Let $S$ be a finite set of integers. Prove that there exists a number $c$ depending on $S$ such that for each non-constant polynomial $f$ with integer coefficients the number of integers $k$ satisfying $f(k)\in S$ does not exceed $\max(\deg f,c)$.
3 replies
jasperE3
May 31, 2021
Bigtaitus
an hour ago
Find the area enclosed by the curve |z|^2 + |z^2 - 2i| = 16
mqoi_KOLA   0
an hour ago
Find the area of the Argand plane enclosed by the curve $$ |z|^2 + |z^2 - 2i| = 16.$$
0 replies
mqoi_KOLA
an hour ago
0 replies
Nice FE from Canada Winter Camp
AshAuktober   1
N 2 hours ago by kokcio
Source: Canada Winter (please provide a link, I can't use search function well on a train)
Find all functions $f:\mathbb{R}\to\mathbb{Z}$ such that $f(x+y)<f(x)+f(y)$ and $f(f(x))=\lfloor x\rfloor+2$ for all reals $x,y$.
1 reply
AshAuktober
3 hours ago
kokcio
2 hours ago
A=b
k2c901_1   85
N 2 hours ago by alexanderhamilton124
Source: Taiwan 1st TST 2006, 1st day, problem 3
Let $a$, $b$ be positive integers such that $b^n+n$ is a multiple of $a^n+n$ for all positive integers $n$. Prove that $a=b$.

Proposed by Mohsen Jamali, Iran
85 replies
k2c901_1
Mar 29, 2006
alexanderhamilton124
2 hours ago
Hard cyclic inequality
JK1603JK   3
N 2 hours ago by arqady
Source: unknown
Prove that $$\frac{a-1}{\sqrt{b+1}}+\frac{b-1}{\sqrt{c+1}}+\frac{c-1}{\sqrt{a+1}}\ge 0,\quad \forall a,b,c>0: a+b+c=3.$$
3 replies
JK1603JK
Today at 4:36 AM
arqady
2 hours ago
Obscure Set Problem
oVlad   1
N 2 hours ago by kokcio
Source: Romania Junior TST 2025 Day 1 P5
Let $n\geqslant 3$ be a positive integer and $\mathcal F$ be a family of at most $n$ distinct subsets of the set $\{1,2,\ldots,n\}$ with the following property: we can consider $n$ distinct points in the plane, labelled $1,2,\ldots,n$ and draw segments connecting these points such that points $i$ and $j$ are connected if and only if $i{}$ belongs to $j$ subsets in $\mathcal F$ for any $i\neq j.$ Determine the maximal value that the sum of the cardinalities of the subsets in $\mathcal{F}$ can take.
1 reply
oVlad
3 hours ago
kokcio
2 hours ago
Congruence
AngleChasingXD   9
N Jan 2, 2022 by Mahdi_Mashayekhi
Source: Tuymaada 2017 Junior Level
$BL $ is the bisector of an isosceles triangle $ABC $. A point $D $ is chosen on the Base $BC $ and a point $E $ is chosen on the lateral side $AB $ so that $AE=\frac {1}{2}AL=CD $. Prove that $LE=LD $.

Tuymaada 2017 Q5 Juniors
9 replies
AngleChasingXD
Jul 18, 2017
Mahdi_Mashayekhi
Jan 2, 2022
Congruence
G H J
G H BBookmark kLocked kLocked NReply
Source: Tuymaada 2017 Junior Level
The post below has been deleted. Click to close.
This post has been deleted. Click here to see post.
AngleChasingXD
109 posts
#1 • 1 Y
Y by Adventure10
$BL $ is the bisector of an isosceles triangle $ABC $. A point $D $ is chosen on the Base $BC $ and a point $E $ is chosen on the lateral side $AB $ so that $AE=\frac {1}{2}AL=CD $. Prove that $LE=LD $.

Tuymaada 2017 Q5 Juniors
This post has been edited 1 time. Last edited by AngleChasingXD, Jul 18, 2017, 5:50 AM
Z K Y
The post below has been deleted. Click to close.
This post has been deleted. Click here to see post.
ThE-dArK-lOrD
4071 posts
#2 • 1 Y
Y by Adventure10
Let $M$ be the midpoint of $AL$, let $\angle{ABL}=\angle{LBC}=x,\angle{MEL}=y$.
Since $\frac{ML}{ME}=\frac{MA}{ME}$, we get $\frac{\sin (y)}{\sin (2x-y)}=\frac{\sin (2x)}{\sin (4x)}=\frac{1}{2\cos (2x)}$. So, $2\cos (2x)\sin (y)=\sin (2x-y)\implies 3\cos (2x)\sin (y)=\sin (2x)\cos (y)$.
We also have $\frac{CD}{CL}=\frac{AL}{2CL}=\frac{AB}{2BC}=\frac{\sin (2x)}{2\sin (4x)}=\frac{1}{4\cos (2x)}$.
Note that $4\sin (y)\cos (2x)=\sin (2x)\cos (y)+\cos (2x)\sin (y)=\sin (2x+y)$.
So $\frac{1}{4\cos (2x)}=\frac{\sin (y)}{\sin (180^{\circ}-2x-y)}=\frac{\sin (\angle{CLD})}{\sin (180^{\circ}-2x-\angle{CLD})}$.
Hence, $\angle{CLD}=y$ and $\angle{CDL}=180^{\circ}-2x-y=\angle{LEB}$.
So, $BELD$ lie on a circle, and so $LE=LD$.
This post has been edited 2 times. Last edited by ThE-dArK-lOrD, Apr 17, 2018, 1:48 PM
Z K Y
The post below has been deleted. Click to close.
This post has been deleted. Click here to see post.
alex_g
26 posts
#3 • 2 Y
Y by Adventure10, Mango247
Project $L$ onto $AB$ and $BC$ at $Q$ and $P$.
Obviously , $LP = LQ$ , $\angle {LQE} = \angle {LPD} = \frac{\pi}{2}$.

After using The Sine Law and The Bisector's Theorem , we get that showing $QE = PD$ is equivalent to showing :

$ 4 \cdot sin^4 (a) + cos (2a) = cos^2 (2a) + 2 \cdot sin^2(a) $ , for some $a$ with $ 0 < a < \frac{\pi}{2} $ ( where $\angle{BAC} = 2a$ ) , which is easy.

Now , $QE = PD$ , so $ \triangle{LQE} = \triangle{LPD} $ , which means that $LE = LD$. Done.
Z K Y
The post below has been deleted. Click to close.
This post has been deleted. Click here to see post.
RagvaloD
4900 posts
#4 • 3 Y
Y by AlastorMoody, Adventure10, Mango247
Let $F$ point on $AB$ such that $LF \parallel BC$
Then $\frac{LF}{BC}=\frac{LA}{AC}$ and $\frac{LC}{BC}=\frac{LA}{AB } \to LF=LC$
Let $M,E$ are midpoints of $AL,AF$. Then $EF=AM=CD$ and $\angle LCD = \angle AFL$ so $\triangle LCD= \triangle EFL$ so $LE=LD$
Z K Y
The post below has been deleted. Click to close.
This post has been deleted. Click here to see post.
fastlikearabbit
28118 posts
#5 • 2 Y
Y by Adventure10, Mango247
AngleChasingXD wrote:
$BL $ is the bisector of an isosceles triangle $ABC $. A point $D $ is chosen on the Base $BC $ and a point $E $ is chosen on the lateral side $AB $ so that $AE=\frac {1}{2}AL=CD $. Prove that $LE=LD $.

Tuymaada 2017 Q5 Juniors

My solution: We know that $AL=\dfrac{b^2}{a+b}, CL=\dfrac{ab}{a+b}$. So $AE=CD=\dfrac{b^2}{2(a+b)}$.
Using law of cosine in $\triangle CDL $ : $ LD^2=\dfrac{(ab)^2}{(a+b)^2}+\dfrac{b^4}{4(a+b)^2}-2\cdot \dfrac{b^2}{2(a+b)} \cdot \dfrac{ab}{a+b} \cdot \cos{C}=\dfrac{b^2(2a^2+b^2)}{4(a+b)^2}$
By law of cosine in $\triangle AEL : LE^2=\dfrac{b^4}{(a+b)^2} +\dfrac{ b^4}{4(a+b)^2} -2 \cdot \dfrac{b^2}{a+b} \cdot \dfrac{b^2}{2(a+b)} \cdot \cos{A} = \dfrac{5b^4-b^4+2a^2b^2}{4(a+b)^2}=\dfrac{b^2(2a^2+b^2)}{4(a+b)^2} $

So $LE=LD$
This post has been edited 1 time. Last edited by fastlikearabbit, Jul 18, 2017, 10:02 AM
Z K Y
The post below has been deleted. Click to close.
This post has been deleted. Click here to see post.
RC.
439 posts
#6 • 1 Y
Y by Adventure10
Why? Why do you guys prefer an ugly method. Draw a circle passing through points \(A , L , D\) to intersect \(BC\) at \(K.\) Use chicken feet theorem. Everything becomes clear I will complete my post :maybe:
This post has been edited 1 time. Last edited by RC., Jul 18, 2017, 11:47 AM
Z K Y
The post below has been deleted. Click to close.
This post has been deleted. Click here to see post.
atmargi
23 posts
#8 • 3 Y
Y by AlastorMoody, Adventure10, Mango247
RC. wrote:
Why? Why do you guys prefer an ugly method. Draw a circle passing through points \(A , L , D\) to intersect \(BC\) at \(K.\) Use chicken feet theorem. Everything becomes clear I will complete my post :maybe:

Lo and behold, the hero never returned.
Z K Y
The post below has been deleted. Click to close.
This post has been deleted. Click here to see post.
Tsikaloudakis
978 posts
#10 • 1 Y
Y by sunken rock
Δείτε το σχήμα:
Attachments:
Z K Y
The post below has been deleted. Click to close.
This post has been deleted. Click here to see post.
arzhang2001
248 posts
#11
Y by
thats enough to prove that $BELD$ is cyclic.
let $N$ be the midpoint of $AC$ . thats enough to prove that $BNLD $is cyclic which this is obvious(by little computing)
This post has been edited 2 times. Last edited by arzhang2001, Nov 2, 2021, 6:30 PM
Z K Y
The post below has been deleted. Click to close.
This post has been deleted. Click here to see post.
Mahdi_Mashayekhi
689 posts
#12
Y by
Let S be on AB such that SL || BC. we will prove triangles LES and LDC are congruent.
CL/LA = BC/BA so CL/BC = LA/BA = AS/AB = SL/BC so SL = CL.
ES = DC , LS = LC and ∠LCD = ∠LSE so LES and LDC are congruent.
now we know LE = LD. we're Done.
Z K Y
N Quick Reply
G
H
=
a