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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.

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[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 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
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Eazy equation clap
giangtruong13   0
4 hours ago
Find all $x,y,z$ satisfy that: $$\frac{x}{y+z}=2x-1; \frac{y}{x+z}=3y-1;\frac{z}{x+y}=5x-1$$
0 replies
giangtruong13
4 hours ago
0 replies
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inequality
revol_ufiaw   3
N 5 hours ago by MS_asdfgzxcvb
Prove that that for any real $x \ge 0$ and natural number $n$,
$$x^n (n+1)^{n+1} \le n^n (x+1)^{n+1}.$$
3 replies
revol_ufiaw
6 hours ago
MS_asdfgzxcvb
5 hours ago
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How to judge a number is prime or not?
mingzhehu   0
5 hours ago
A=（10X1+1）(10X+1），X1，X∈N+
B=（10 X1+3）（10X+7），X∈N，X1∈N
C=(10 X1+9)(10X+9), X∈N，X1∈N
D=(10 X1+1)(10X+3), X1∈N+，X∈N
E=（10 X1+7）(10X+9），X∈N，X1∈N
F=（10 X1+1）（10X+7），X1∈N+，X∈N
G=（10 X1+3）（10X+9），X∈N，X1∈N
H=（10 X1+1）10X+9），X1∈N+，X∈N
I=（10 X1+3）（10X+3），X1∈N，X∈N
J=( 10X1+7)(10X+7),X∈N，X1∈N

For any natural number P∈{P=10N+1，n∈N}，make P=A or B or C
If P can make the roots of function group(ABC) without any root group completely made up of integer, P will be a prime
For any natural number P∈{P=10N+3，n∈N}，make P=D or E
If P can make the roots of function group(DE) without any root group completely made up
of integer, P will be a prime
For any natural number P∈{P=10N+7，n∈N}，make P=F or G
If P can make the roots of function group(FG) without any root group completely made up
of integer, P will be a prime
For any natural number P∈{P=10N+9，n∈N}，make P=H or I or J
If P can make the roots of function group(GIJ) without any root group completely made up
of integer, P will be a prime
0 replies
mingzhehu
5 hours ago
0 replies
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What is an isogonal conjugate and why is it useful?
EaZ_Shadow   6
N 5 hours ago by maxamc
What is an isogonal conjugate and why is it useful? People use them in Olympiad geometry proofs but I don’t understand why and what is the purpose, as it complicates me because of me not understanding it.
6 replies
EaZ_Shadow
Dec 28, 2024
maxamc
5 hours ago
J
No more topics!
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isosceles criterion, equal perpσ on bisectors (Ukr. Geom. Olympiad '20 VIII p1)
parmenides51   1
N Jun 8, 2020 by Inconsistent
In triangle $ABC$, bisectors are drawn $AA_1$ and $CC_1$. Prove that if the length of the perpendiculars drawn from the vertex $B$ on lines $AA1$ and $CC_1$ are equal, then $\vartriangle ABC$ is isosceles.
1 reply
parmenides51
Jun 8, 2020
Inconsistent
Jun 8, 2020
J
isosceles criterion, equal perpσ on bisectors (Ukr. Geom. Olympiad '20 VIII p1)
G H J
Reveal topic tags
geometry
isosceles
angle bisector
perpendicular
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parmenides51
30629 posts
parmenides51
#1 h  Jun 8, 2020, 9:06 PM
Y by
In triangle $ABC$, bisectors are drawn $AA_1$ and $CC_1$. Prove that if the length of the perpendiculars drawn from the vertex $B$ on lines $AA1$ and $CC_1$ are equal, then $\vartriangle ABC$ is isosceles.
This post has been edited 1 time. Last edited by parmenides51, Jun 8, 2020, 9:37 PM
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Inconsistent
1455 posts
Inconsistent
#2 h  Jun 8, 2020, 9:20 PM
Y by
If the distance from a point to two intersecting lines is equal then it is on one of the bisectors of those two lines. Note that B must be on the bisector that splits A and C because otherwise, the incenter will be outside the triangle! Now we already know that for the angles around I next to the ray IB to be equal, 90-<A/2=90-<C/2 so <A=<C, it is isoceles.
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