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perpendicularity involving ex and incenter
Erken   19
N 19 minutes ago by Primeniyazidayi
Source: Kazakhstan NO 2008 problem 2
Suppose that $ B_1$ is the midpoint of the arc $ AC$, containing $ B$, in the circumcircle of $ \triangle ABC$, and let $ I_b$ be the $ B$-excircle's center. Assume that the external angle bisector of $ \angle ABC$ intersects $ AC$ at $ B_2$. Prove that $ B_2I$ is perpendicular to $ B_1I_B$, where $ I$ is the incenter of $ \triangle ABC$.
19 replies
Erken
Dec 24, 2008
Primeniyazidayi
19 minutes ago
Partitioning a set so that sum of powers is balanced
Miquel-point   0
21 minutes ago
Source: Romanina IMO TST 1981, Day 3 P2
Show that a set $A$ consisting of $16$ consecutive non-negative integers can be partitioned in two disjoint sets $X$ and $Y$ each containing $8$ elements so that \(\sum\limits_{x\in X}x^k=\sum\limits_{y\in Y} y^k,\) for $k=1,2,3.$
0 replies
Miquel-point
21 minutes ago
0 replies
Sum of max of some sequences is sum of powers
Miquel-point   0
24 minutes ago
Source: Romanian IMO TST 1981, Day 3 P1
Consider the set $M$ of all sequences of integers $s=(s_1,\ldots,s_k)$ such that $0\leqslant s_i\leqslant n,\; i=1,2,\ldots,k$ and let $M(s)=\max\{s_1,\ldots,s_k\}$. Show that
\[\sum_{s\in A} M(s)=(n+1)^{k+1}-(1^k+2^k+\ldots +(n+1)^k).\]
Ioan Tomescu
0 replies
Miquel-point
24 minutes ago
0 replies
Locus of sphere cutting three spheres along great circles
Miquel-point   0
28 minutes ago
Source: Romanian IMO TST 1981, Day 2 P3
Consider three fixed spheres $S_1, S_2, S_3$ with pairwise disjoint interiors. Determine the locus of the centre of the sphere intersecting each $S_i$ along a great circle of $S_i$.

Stere Ianuș
0 replies
Miquel-point
28 minutes ago
0 replies
s(n) and s(n+1) divisible by m
Miquel-point   0
31 minutes ago
Source: Romanian IMO TST 1981, Day 2 P2
Let $m$ be a positive integer not divisible by 3. Prove that there are infinitely many positive integers $n$ such that $s(n)$ and $s(n+1)$ are divisible by $m$, where $s(x)$ is the sum of digits of $x$.

Dorel Miheț
0 replies
Miquel-point
31 minutes ago
0 replies
One of first p terms in sequence isn't prime
Miquel-point   1
N 31 minutes ago by Filipjack
Source: Romanian IMO TST 1981, Day 1 P3
Let $p>2$ be a prime number, and $(a_k)_{k\geqslant 1}$ be a sequence defined by $a_1=p$ and $a_{k+1}=2a_k+1$, $k\geqslant 1$. Show that one of the first $p$ terms of the sequence is not prime.

Marcel Țena
1 reply
Miquel-point
40 minutes ago
Filipjack
31 minutes ago
max |sin x|, |sin (x+1)| > 1/3
Miquel-point   0
33 minutes ago
Source: Romanian IMO TST 1981, Day 2 P1
Show that for every real number $x$ we have
\[\max(|\sin x|,|\sin (x+1)|)>\frac13.\]
0 replies
Miquel-point
33 minutes ago
0 replies
Colouring lattice points from 1981
Miquel-point   0
36 minutes ago
Source: Romanian IMO TST 1981, Day 1 P5
Consider the set $S$ of lattice points with positive coordinates in the plane. For each point $P(a,b)$ from $S$, we draw a segment between it and each of the points in the set \[S(P)=\{(a+b,c)\mid c\in\mathbb{Z}, \, c>a+b\}.\]Show that there is no colouring of the points in $S$ with a finite number of colours such that every two points joined by a segment are coloured with different colours.

Ioan Tomescu
0 replies
Miquel-point
36 minutes ago
0 replies
max(PA,PC) when ABCD square
Miquel-point   0
42 minutes ago
Source: Romanian IMO TST 1981, P2 Day 1
Determine the set of points $P$ in the plane of a square $ABCD$ for which \[\max (PA, PC)=\frac1{\sqrt2}(PB+PD).\]
Titu Andreescu and I.V. Maftei
0 replies
Miquel-point
42 minutes ago
0 replies
Polynomial divisible by x^2+1
Miquel-point   0
44 minutes ago
Source: Romanian IMO TST 1981, P1 Day 1
Consider the polynomial $P(X)=X^{p-1}+X^{p-2}+\ldots+X+1$, where $p>2$ is a prime number. Show that if $n$ is an even number, then the polynomial \[-1+\prod_{k=0}^{n-1} P\left(X^{p^k}\right)\]is divisible by $X^2+1$.

Mircea Becheanu
0 replies
Miquel-point
44 minutes ago
0 replies
2000th post!
evt917   35
N Mar 10, 2025 by eddie.li
Wow I can't believe I'm at 2000 posts already! I guess this also celebrates my (late) 3 year anniversary on AoPS!

um i guess i share my story (most problems are written by me)

1st grade -- i forgot ok

2nd grade -- i was at public school like the regular kids, there i started loving basketball and i was already working on prealgebra level stuff example problem

3rd grade -- started aops i skipped to calculus anyway so here i was still in public school and I started algebra a, but made no progress so my parents asked me if I want to homeschool (lucky they didn't force me), and i said yes example problem

4th grade -- Finished intro to programming with python, got 13 on amc 8 (skull), finished all intro courses except intro to number theory. i was shaky on lots of concepts and i had to do a review (by myself with the aops books) sometimes. example problem

5th grade -- finished intermediate programming with python but somehow failed usaco (the score shall be undisclosed), i started learning some basic C++, and finished all intro courses and im doing intermediate algebra and intermediate number theory now :D . I got 21 on amc 8 (improvement but no dhr) and 72 on amc 10a (buh). oh and by the way I'm still playing basketball in my rec league :) example problem

for more example problems go to my two mock amc 8's and keep an eye on my other mock im about to create!

anyway thanks aops for 2000 posts it helped me learn so much


p.s. lots of information i didn't share but this is the general idea (also pls upvote! if i reach 10 upvotes i will create something special here)
35 replies
evt917
Feb 24, 2025
eddie.li
Mar 10, 2025
2000th post!
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evt917
2257 posts
#1 • 13 Y
Y by DarintheBoy, goldendog, froyocat, Creeperboat, Squidget, HWenslawski, Sedro, Yrock, jkim0656, fastgoodboy, BookExplorer, Yiyj1, sangriabeaver5
Wow I can't believe I'm at 2000 posts already! I guess this also celebrates my (late) 3 year anniversary on AoPS!

um i guess i share my story (most problems are written by me)

1st grade -- i forgot ok

2nd grade -- i was at public school like the regular kids, there i started loving basketball and i was already working on prealgebra level stuff example problem

3rd grade -- started aops i skipped to calculus anyway so here i was still in public school and I started algebra a, but made no progress so my parents asked me if I want to homeschool (lucky they didn't force me), and i said yes example problem

4th grade -- Finished intro to programming with python, got 13 on amc 8 (skull), finished all intro courses except intro to number theory. i was shaky on lots of concepts and i had to do a review (by myself with the aops books) sometimes. example problem

5th grade -- finished intermediate programming with python but somehow failed usaco (the score shall be undisclosed), i started learning some basic C++, and finished all intro courses and im doing intermediate algebra and intermediate number theory now :D . I got 21 on amc 8 (improvement but no dhr) and 72 on amc 10a (buh). oh and by the way I'm still playing basketball in my rec league :) example problem

for more example problems go to my two mock amc 8's and keep an eye on my other mock im about to create!

anyway thanks aops for 2000 posts it helped me learn so much


p.s. lots of information i didn't share but this is the general idea (also pls upvote! if i reach 10 upvotes i will create something special here)
This post has been edited 2 times. Last edited by evt917, Feb 26, 2025, 4:27 AM
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orangebear
661 posts
#2 • 1 Y
Y by HWenslawski
Congrats! :clap: It will be a long time till I get my 2000th post.
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jocaleby1
193 posts
#3 • 1 Y
Y by HWenslawski
Congrats on 2000 posts!
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froyocat
1065 posts
#4 • 1 Y
Y by HWenslawski
bruh nice
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derekwang2048
1207 posts
#5 • 1 Y
Y by HWenslawski
nice you're in my class
and made me lose 157 rating on chess.com >:(
also the last problem is from HMMT and I saw on ftw
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happymoose666
294 posts
#6 • 1 Y
Y by HWenslawski
wow 2000 posts and in 6th grade???
Im in 7th and I got 23 in amc8s, 70 amc10a
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GlitchyBoy
86 posts
#7 • 1 Y
Y by HWenslawski
wow yall are smart, im in 7th and i got 16 on amc8, 58.5 amc10b
got 33 on chapter last yr as a 6th grader, dont wanna disclose my score this year cuz i did worse:)
congratulations on 2000 posts (coming from a 20 post ahh guy)
This post has been edited 1 time. Last edited by GlitchyBoy, Feb 25, 2025, 4:04 AM
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jb2015007
1807 posts
#8 • 1 Y
Y by HWenslawski
problem for grade 5 DEFINITELY not copied off of intro to nt textbook...
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evt917
2257 posts
#9 • 1 Y
Y by HWenslawski
ok well i written too much competitions im out of ideas
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CC_chloe
308 posts
#10 • 1 Y
Y by HWenslawski
congrats pro this is my 300th :P
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LittleBacat
13 posts
#11 • 1 Y
Y by HWenslawski
congrats!
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giselezhu
615 posts
#12 • 1 Y
Y by HWenslawski
@LittleBacat, you could get more posts! (I think i have 529 or around something like that)
Also, congrats to @evt917!!!!! :clap2: :clap2: :clap2:
I’ve did aops for around 1/2 a yr i think?

So basically this is my math life

In Grade 1 (year 2) i was working on stuff on geometry like there’s 2 overlapping squares and you calculate a shaded area with 3 given side lengths
In Grade 2 (year 3) i focused on doing more variable work and learnt the Pythagorean theorem
In Grade 3 (year 4) i was SUPER bored because the school work was tooooo easy and tried to understand the proof of the Pythagorean theorem, which wasn’t easy at that time while also doing ratios
Grade 4 (year 5) joined aops and finally understood Pythagorean theorem learned to calculate area of a circle, doing AMC 8, and reading books about basic trigonometry

Rnow i only need 1 min to do school math homework (about)

This year I got 16 for AMC 8 i think not bad because I’m only 4th grade!!!!! I can’t really find anyone on aops the same age as me. Also i type fastest in my whole class!
This post has been edited 5 times. Last edited by giselezhu, Feb 25, 2025, 11:15 AM
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giangtruong13
98 posts
#13 • 1 Y
Y by HWenslawski
Lol, what's the sigma
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littlebigbull
498 posts
#14 • 1 Y
Y by HWenslawski
freeal
freeal
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Yrock
1348 posts
#15 • 1 Y
Y by HWenslawski
giselezhu wrote:
@LittleBacat, you could get more posts! (I think i have 529 or around something like that)
Also, congrats to @evt917!!!!! :clap2: :clap2: :clap2:
I’ve did aops for around 1/2 a yr i think?

So basically this is my math life

In Grade 1 (year 2) i was working on stuff on geometry like there’s 2 overlapping squares and you calculate a shaded area with 3 given side lengths
In Grade 2 (year 3) i focused on doing more variable work and learnt the Pythagorean theorem
In Grade 3 (year 4) i was SUPER bored because the school work was tooooo easy and tried to understand the proof of the Pythagorean theorem, which wasn’t easy at that time while also doing ratios
Grade 4 (year 5) joined aops and finally understood Pythagorean theorem learned to calculate area of a circle, doing AMC 8, and reading books about basic trigonometry

Rnow i only need 1 min to do school math homework (about)

This year I got 16 for AMC 8 i think not bad because I’m only 4th grade!!!!! I can’t really find anyone on aops the same age as me. Also i type fastest in my whole class!

I'm 4th grade too lol

also gj on 2000 posts! I've been here for 3 years almost and I only have 1200...
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dionysusheero
33 posts
#16 • 1 Y
Y by HWenslawski
Congrats
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sadas123
1178 posts
#17
Y by
Congrats!!!!! I was defeinetley not good as you last year in 5th grade I got a 12 on AMC 8 :(
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RedChameleon
1226 posts
#18 • 1 Y
Y by fastgoodboy
evt917 wrote:
Wow I can't believe I'm at 2000 posts already! I guess this also celebrates my (late) 2 year anniversary on AoPS!

Three year anniversary my guy

Three
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Yrock
1348 posts
#19 • 1 Y
Y by fastgoodboy
evt917 wrote:
(all problems are written by me)
jb2015007 wrote:
problem for grade 5 DEFINITELY not copied off of intro to nt textbook...

I am confused lol
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fastgoodboy
310 posts
#20
Y by
I'm a long way to 2000
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MathBishop
33 posts
#21
Y by
same here
fastgoodboy wrote:
I'm a long way to 2000

:noo:
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Yiyj1
1229 posts
#22
Y by
more than halfway there im on the grind
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evt917
2257 posts
#23
Y by
okay 10 upvotes! I promised to post something special...

It'll be ready in really early march!
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evt917
2257 posts
#24
Y by
also it took wayy less time to get a 1000 posts to start than to get my next 1000 posts...
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RoyalPrince
186 posts
#25 • 1 Y
Y by BookExplorer
Now I'm aiming for 2000 posts eventually. Click to reveal hidden text
This post has been edited 7 times. Last edited by RoyalPrince, Feb 26, 2025, 4:11 PM
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evt917
2257 posts
#27
Y by
okay, the something special i'll post is... my 1000th post on AoPS! I took so long finding it
Attachments:
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Jello0211
12 posts
#28
Y by
good job
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valisaxieamc
67 posts
#29
Y by
evt917 wrote:
also it took wayy less time to get a 1000 posts to start than to get my next 1000 posts...

How long did it take you to make your first 1000 posts?
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evt917
2257 posts
#30
Y by
idk?

we're not done; here is my first post lol
Attachments:
This post has been edited 1 time. Last edited by evt917, Mar 5, 2025, 3:23 AM
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Yiyj1
1229 posts
#31
Y by
evt917 wrote:
idk?

we're not done; here is my first post lol

lol that's an iinteresting first post
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evt917
2257 posts
#32
Y by
it took me until March 2024 to get my 1000th post
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eddie.li
24 posts
#33
Y by
why did you go too algebra a though?
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eddie.li
24 posts
#34
Y by
evt917 wrote:
idk?

we're not done; here is my first post lol

Lol
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JohannIsBach
126 posts
#35
Y by
nice! i want that much! :wow:
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valisaxieamc
67 posts
#36
Y by
sadas123 wrote:
Congrats!!!!! I was defeinetley not good as you last year in 5th grade I got a 12 on AMC 8 :(

I got a 6 back in 5th grade lol
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eddie.li
24 posts
#37
Y by
I’m in second grade and am already in algebra a but I don’t have many posts
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