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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
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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]
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0 replies
jlacosta
Apr 2, 2025
0 replies
Inspired by hlminh
sqing   0
a minute ago
Source: Own
Let $ a,b,c $ be real numbers such that $ a^2+b^2+c^2=1. $ Prove that $$ |a-kb|+|b-kc|+|c-ka|\leq \sqrt{3k^2+2k+3}$$Where $ k\geq 0 . $
0 replies
1 viewing
sqing
a minute ago
0 replies
Is this FE solvable?
ItzsleepyXD   3
N 14 minutes ago by jasperE3
Source: Original
Let $c_1,c_2 \in \mathbb{R^+}$. Find all $f : \mathbb{R^+} \rightarrow \mathbb{R^+}$ such that for all $x,y \in \mathbb{R^+}$ $$f(x+c_1f(y))=f(x)+c_2f(y)$$
3 replies
ItzsleepyXD
Yesterday at 3:02 AM
jasperE3
14 minutes ago
PQ bisects AC if <BCD=90^o, A, B,C,D concyclic
parmenides51   2
N 19 minutes ago by venhancefan777
Source: Mathematics Regional Olympiad of Mexico Northeast 2020 P2
Let $A$, $B$, $C$ and $D$ be points on the same circumference with $\angle BCD=90^\circ$. Let $P$ and $Q$ be the projections of $A$ onto $BD$ and $CD$, respectively. Prove that $PQ$ cuts the segment $AC$ into equal parts.
2 replies
parmenides51
Sep 7, 2022
venhancefan777
19 minutes ago
Inequality with three conditions
oVlad   3
N 21 minutes ago by sqing
Source: Romania EGMO TST 2019 Day 1 P3
Let $a,b,c$ be non-negative real numbers such that \[b+c\leqslant a+1,\quad c+a\leqslant b+1,\quad a+b\leqslant c+1.\]Prove that $a^2+b^2+c^2\leqslant 2abc+1.$
3 replies
oVlad
Yesterday at 1:48 PM
sqing
21 minutes ago
No more topics!
5 concyclic points defined by equal angles
parmenides51   4
N Mar 27, 2019 by Psimo
Source: Mathematical Ashes 2017
Point $A_1$ lies inside acute scalene triangle $ABC$ and satisfies $\angle A_1AB = \angle A_1BC$ and $\angle A_1AC = \angle A_1CB$. Points $B_1$ and $C_1$ are similarly defined. Let $G$ and $H$ be the centroid and orthocentre, repsectively, of triangle $ABC$. Prove that $A_1, B_1, C_1, G$, and $H$ all lie on a common circle.
4 replies
parmenides51
Jul 1, 2018
Psimo
Mar 27, 2019
5 concyclic points defined by equal angles
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G H BBookmark kLocked kLocked NReply
Source: Mathematical Ashes 2017
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parmenides51
30630 posts
#1 • 2 Y
Y by Adventure10, Mango247
Point $A_1$ lies inside acute scalene triangle $ABC$ and satisfies $\angle A_1AB = \angle A_1BC$ and $\angle A_1AC = \angle A_1CB$. Points $B_1$ and $C_1$ are similarly defined. Let $G$ and $H$ be the centroid and orthocentre, repsectively, of triangle $ABC$. Prove that $A_1, B_1, C_1, G$, and $H$ all lie on a common circle.
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H.HAFEZI2000
328 posts
#2 • 2 Y
Y by Adventure10, Mango247
there is a lemmas which will help a lot

lemma : $HA_1$ is perpendicular to $AM$ that $M$ is the midpoint of $BC$


And the right angles $\angle HA_1G=90$ and $\angle HB_1G=90$ and $\angle HC_1G=90$ tells and $A_1, B_1 and C_1$ lies on a circle with diameter GH

another interesting related fact: if $\overline{M}$ was the intersection of circumcircle of ABC and AM then $A_1$ is on line AM and also $\overline{M}$$M=MA_1$
This post has been edited 2 times. Last edited by H.HAFEZI2000, Jul 2, 2018, 12:18 AM
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enhanced
515 posts
#3 • 2 Y
Y by Adventure10, Mango247
$A_1,B_1,C_1$ are the humpty points so the rest is simple angle chasing .
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Psimo
29 posts
#4 • 1 Y
Y by Adventure10
can the properties of humpty points be used directly without providing any proof ?
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Psimo
29 posts
#5 • 1 Y
Y by Adventure10
enhanced wrote:
$A_1,B_1,C_1$ are the humpty points so the rest is simple angle chasing .
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