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Problem 1
SpectralS   145
N 6 minutes ago by IndexLibrorumProhibitorum
Given triangle $ABC$ the point $J$ is the centre of the excircle opposite the vertex $A.$ This excircle is tangent to the side $BC$ at $M$, and to the lines $AB$ and $AC$ at $K$ and $L$, respectively. The lines $LM$ and $BJ$ meet at $F$, and the lines $KM$ and $CJ$ meet at $G.$ Let $S$ be the point of intersection of the lines $AF$ and $BC$, and let $T$ be the point of intersection of the lines $AG$ and $BC.$ Prove that $M$ is the midpoint of $ST.$

(The excircle of $ABC$ opposite the vertex $A$ is the circle that is tangent to the line segment $BC$, to the ray $AB$ beyond $B$, and to the ray $AC$ beyond $C$.)

Proposed by Evangelos Psychas, Greece
145 replies
SpectralS
Jul 10, 2012
IndexLibrorumProhibitorum
6 minutes ago
Integrable function: + and - on every subinterval.
SPQ   3
N Today at 7:06 AM by solyaris
Provide a function integrable on [a, b] such that f takes on positive and negative values on every subinterval (c, d) of [a, b]. Prove your function satisfies both conditions.
3 replies
SPQ
Today at 2:40 AM
solyaris
Today at 7:06 AM
Putnam 1999 A4
djmathman   7
N Today at 7:05 AM by P162008
Sum the series \[\sum_{m=1}^\infty\sum_{n=1}^\infty\dfrac{m^2n}{3^m(n3^m+m3^n)}.\]
7 replies
djmathman
Dec 22, 2012
P162008
Today at 7:05 AM
Find the greatest possible value of the expression
BEHZOD_UZ   1
N Today at 6:34 AM by alexheinis
Source: Yandex Uzbekistan Coding and Math Contest 2025
Let $a, b, c, d$ be complex numbers with $|a| \le 1, |b| \le 1, |c| \le 1, |d| \le 1$. Find the greatest possible value of the expression $$|ac+ad+bc-bd|.$$
1 reply
BEHZOD_UZ
Yesterday at 11:56 AM
alexheinis
Today at 6:34 AM
Problem vith lcm
snowhite   2
N Today at 6:21 AM by snowhite
Prove that $\underset{n\to \infty }{\mathop{\lim }}\,\sqrt[n]{lcm(1,2,3,...,n)}=e$
Please help me! Thank you!
2 replies
snowhite
Today at 5:19 AM
snowhite
Today at 6:21 AM
combinatorics
Hello_Kitty   2
N Yesterday at 10:23 PM by Hello_Kitty
How many $100$ digit numbers are there
- not including the sequence $123$ ?
- not including the sequences $123$ and $231$ ?
2 replies
Hello_Kitty
Apr 17, 2025
Hello_Kitty
Yesterday at 10:23 PM
Sequence of functions
Squeeze   2
N Yesterday at 10:22 PM by Hello_Kitty
Q) let $f_n:[-1,1)\to\mathbb{R}$ and $f_n(x)=x^{n}$ then is this uniformly convergence on $(0,1)$ comment on uniformly convergence on $[0,1]$ where in general it is should be uniformly convergence.

My I am trying with some contradicton method like chose $\epsilon=1$ and trying to solve$|f_n(a)-f(a)|<\epsilon=1$
Next take a in (0,1) and chose a= 2^1/N but not solution
How to solve like this way help.
2 replies
Squeeze
Apr 18, 2025
Hello_Kitty
Yesterday at 10:22 PM
A in M2(prime), A=B^2 and det(B)=p^2
jasperE3   1
N Yesterday at 9:59 PM by KAME06
Source: VJIMC 2012 1.2
Determine all $2\times2$ integer matrices $A$ having the following properties:

$1.$ the entries of $A$ are (positive) prime numbers,
$2.$ there exists a $2\times2$ integer matrix $B$ such that $A=B^2$ and the determinant of $B$ is the square of a prime number.
1 reply
jasperE3
May 31, 2021
KAME06
Yesterday at 9:59 PM
Equation over a finite field
loup blanc   1
N Yesterday at 9:30 PM by alexheinis
Find the set of $x\in\mathbb{F}_{5^5}$ such that the equation in the unknown $y\in \mathbb{F}_{5^5}$:
$x^3y+y^3+x=0$ admits $3$ roots: $a,a,b$ s.t. $a\not=b$.
1 reply
loup blanc
Yesterday at 6:08 PM
alexheinis
Yesterday at 9:30 PM
Integration Bee Kaizo
Calcul8er   51
N Yesterday at 7:41 PM by BaidenMan
Hey integration fans. I decided to collate some of my favourite and most evil integrals I've written into one big integration bee problem set. I've been entering integration bees since 2017 and I've been really getting hands on with the writing side of things over the last couple of years. I hope you'll enjoy!
51 replies
Calcul8er
Mar 2, 2025
BaidenMan
Yesterday at 7:41 PM
interesting integral
Martin.s   1
N Yesterday at 2:46 PM by ysharifi
$$\int_0^\infty \frac{\sinh(t)}{t \cosh^3(t)} dt$$
1 reply
Martin.s
Monday at 3:12 PM
ysharifi
Yesterday at 2:46 PM
T lies on Euler circle
ken3k06   6
N Jun 1, 2022 by jayme
Let $\displaystyle ABC$ be a triangle with altitudes $\displaystyle AD,BE,CF$ intersect at orthocenter $\displaystyle H$. Let $\displaystyle T$ be an arbitrary point lies on $\displaystyle ( DEF)$. $\displaystyle D'$ be the reflection point of $\displaystyle D$ on $\displaystyle AT$. $\displaystyle D'T$ intersects $\displaystyle EF$ at $\displaystyle P$. Prove that $\displaystyle ( DTP)$ passes through the circumcenter of $\displaystyle ADP$.

IMAGE
6 replies
ken3k06
May 25, 2022
jayme
Jun 1, 2022
T lies on Euler circle
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ken3k06
424 posts
#1
Y by
Let $\displaystyle ABC$ be a triangle with altitudes $\displaystyle AD,BE,CF$ intersect at orthocenter $\displaystyle H$. Let $\displaystyle T$ be an arbitrary point lies on $\displaystyle ( DEF)$. $\displaystyle D'$ be the reflection point of $\displaystyle D$ on $\displaystyle AT$. $\displaystyle D'T$ intersects $\displaystyle EF$ at $\displaystyle P$. Prove that $\displaystyle ( DTP)$ passes through the circumcenter of $\displaystyle ADP$.

https://scontent.fdad3-1.fna.fbcdn.net/v/t1.15752-9/280650918_1427756197694764_7145749033231498711_n.png?_nc_cat=103&ccb=1-7&_nc_sid=ae9488&_nc_ohc=FnjTOESeskgAX_1wK43&_nc_ht=scontent.fdad3-1.fna&oh=03_AVLcP-KSAuX1yT9QQnr-7uA5KQKqpYNK3FAoMz3bkSIasQ&oe=62B1F047
This post has been edited 1 time. Last edited by ken3k06, May 25, 2022, 1:38 AM
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jayme
9782 posts
#2 • 1 Y
Y by ken3k06
Any ideas?

Sincerely
Jean-Louis
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VZH
60 posts
#3 • 1 Y
Y by ken3k06
Hints:
We only need to prove $\angle PTD= 2\angle PAD$, and because $D$ and $D'$ are symmetric wrt $AT$, it suffices to show $(ATP)$ is tangent to $AD$.
Consider an inversion wrt $A$ that swaps $(B,F)$ and $(D,E)$. Now, this inversion sends $P, T$ to $P', T'$, where $P'$ and $T'$ lie on $(ABC)$ and $(BHC)$ respectively. Let $H'$ be the reflection of $H$ on $AT$, then the inversion also swaps $(H',D')$, and so $A,T',G',H'$ are concyclic. From here, proving $T'G' \parallel AD$ is easy.
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jayme
9782 posts
#4 • 2 Y
Y by ken3k06, Mango247
Dear,
thank you for your proof...
Do you have an idea for a synthetic proof without inversion?

Sincerely
Jean-Louis
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jayme
9782 posts
#5 • 1 Y
Y by ken3k06
Bump!

Sincerely
Jean-Louis
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jayme
9782 posts
#6 • 1 Y
Y by ken3k06
last Bump!

Sincerely
Jean-Louis
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jayme
9782 posts
#7
Y by
Dear Mathlinkersn
finally I have found a proof...

Any references?

Thank in advance
Sincerely
Jean-Louis
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