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IMO Shortlist 2011, G4
WakeUp   125
N 8 minutes ago by Davdav1232
Source: IMO Shortlist 2011, G4
Let $ABC$ be an acute triangle with circumcircle $\Omega$. Let $B_0$ be the midpoint of $AC$ and let $C_0$ be the midpoint of $AB$. Let $D$ be the foot of the altitude from $A$ and let $G$ be the centroid of the triangle $ABC$. Let $\omega$ be a circle through $B_0$ and $C_0$ that is tangent to the circle $\Omega$ at a point $X\not= A$. Prove that the points $D,G$ and $X$ are collinear.

Proposed by Ismail Isaev and Mikhail Isaev, Russia
125 replies
WakeUp
Jul 13, 2012
Davdav1232
8 minutes ago
Putnam 1972 A2
sqrtX   2
N 4 hours ago by KAME06
Source: Putnam 1972
Let $S$ be a set with a binary operation $\ast$ such that
1) $a \ast(a\ast b)=b$ for all $a,b\in S$.
2) $(a\ast b)\ast b=a$ for all $a,b\in S$.
Show that $\ast$ is commutative and give an example where $\ast$ is not associative.
2 replies
sqrtX
Feb 17, 2022
KAME06
4 hours ago
Limit with sin^2x
Quantum_fluctuations   7
N Today at 7:25 AM by P162008

Evaluate:

$\lim_{x \to 0} \left( 1^{1/\sin^2 x} + 2^{1/\sin^2 x} + 3^{1/\sin^2 x} + .  .  . + n^{1/\sin^2 x} \right)^{\sin^2 x}$
7 replies
Quantum_fluctuations
Apr 26, 2020
P162008
Today at 7:25 AM
Decimal number defined recursively by digit sums modulo 10
fermion13pi   2
N Today at 7:20 AM by solyaris
Source: Competição Elon Lages Lima
Consider the real number written in decimal notation:
r = 0.235831...
where, starting from the third digit after the decimal point, each digit is equal to the remainder when the sum of the previous two digits is divided by 10.

Which of the following statements is true?

(a) (10⁶⁰ - 1).r is an integer
(b) (10²⁵ - 1).r is an integer
(c) (10¹⁷ - 1).r is an integer
(d) r is an irrational algebraic number
(e) r is an irrational transcendental number

(Recall that a complex number is called algebraic if it is a root of a non-zero polynomial with integer coefficients.)
2 replies
fermion13pi
Yesterday at 11:14 PM
solyaris
Today at 7:20 AM
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 with 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
1 area = 2025 points
giangtruong13   1
N Apr 4, 2025 by kiyoras_2001
In a plane give a set $H$ that has 8097 distinct points with area of a triangle that has 3 points belong to $H$ all $ \leq 1$. Prove that there exists a triangle $G$ that has the area $\leq 1 $ contains at least 2025 points that belong to $H$( each of that 2025 points can be inside the triangle or lie on the edge of triangle $G$)X
1 reply
giangtruong13
Apr 4, 2025
kiyoras_2001
Apr 4, 2025
1 area = 2025 points
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giangtruong13
128 posts
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In a plane give a set $H$ that has 8097 distinct points with area of a triangle that has 3 points belong to $H$ all $ \leq 1$. Prove that there exists a triangle $G$ that has the area $\leq 1 $ contains at least 2025 points that belong to $H$( each of that 2025 points can be inside the triangle or lie on the edge of triangle $G$)X
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kiyoras_2001
674 posts
#2
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Replace $2025$ with $k$ and $8097$ with $4k-3$. Consider the triangle $ABC$ of maximal area with vertices among the $4k+3$ points. Let $A_1$ be such that $ABA_1C$ is a parallelogram. SImilarly define $B_1, C_1$. Then all the $4k-3$ points lie in the closed triangle $A_1B_1C_1$ (why?). Then one of the closed triangles $ABC, A_1BC, AB_1C, ABC_1$ contains at least $k$ points as desired (if each of them contains at most $k-1$ points then overall would be at most $4k-4$ points).
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