Found problems: 85335
2020 Final Mathematical Cup, 3
Given a paper on which the numbers $1,2,3\dots ,14,15$ are written. Andy and Bobby are bored and perform the following operations, Andy chooses any two numbers (say $x$ and $y$) on the paper, erases them, and writes the sum of the numbers on the initial paper. Meanwhile, Bobby writes the value of $xy(x+y)$ in his book. They were so bored that they both performed the operation until only $1$ number remained. Then Bobby adds up all the numbers he wrote in his book, let’s call $k$ as the sum.
$a$. Prove that $k$ is constant which means it does not matter how they perform the operation,
$b$. Find the value of $k$.
2007 Moldova National Olympiad, 12.1
For $a\in C^{*}$ find all $n\in N$ such that $X^{2}(X^{2}-aX+a^{2})^{2}$ divides $(X^{2}+a^{2})^{n}-X^{2n}-a^{2n}$
1993 Czech And Slovak Olympiad IIIA, 6
Show that there exists a tetrahedron which can be partitioned into eight congruent tetrahedra, each of which is similar to the original one.
1997 AMC 8, 5
There are many two-digit multiples of 7, but only two of the multiples have a digit sum of 10. The sum of these two multiples of 7 is
$\textbf{(A)}\ 119 \qquad \textbf{(B)}\ 126 \qquad \textbf{(C)}\ 140 \qquad \textbf{(D)}\ 175 \qquad \textbf{(E)}\ 189$
2016 PUMaC Team, 2
Temerant is a spherical planet with radius $1000$ kilometers. The government wants to build twelve towers of the same height on the equator of Temerant, so that every point on the equator can be seen from at least one tower. The minimum possible height of the towers can be written, in kilometers, as a $\sqrt{b} - c\sqrt{d} - e$ for positive integers $a, b, c, d$, and $e$ (with $b$ and $d$ not divisible by the square of any prime). Compute $a + b + c + d + e$.
2004 AMC 10, 25
Three mutually tangent spheres of radius 1 rest on a horizontal plane. A sphere of radius 2 rests on them. What is the distance from the plane to the top of the larger sphere?
$ \textbf{(A)}\; 3+\frac{\sqrt{30}}2\qquad
\textbf{(B)}\; 3+\frac{\sqrt{69}}3\qquad
\textbf{(C)}\; 3+\frac{\sqrt{123}}4\qquad
\textbf{(D)}\; \frac{52}9\qquad
\textbf{(E)}\; 3+2\sqrt{2} $
Indonesia Regional MO OSP SMA - geometry, 2007.1
Let $ABCD$ be a quadrilateral with $AB = BC = CD = DA$.
(a) Prove that point A must be outside of triangle $BCD$.
(b) Prove that each pair of opposite sides on $ABCD$ is always parallel.
1998 All-Russian Olympiad, 2
Let $ABC$ be a triangle with circumcircle $w$. Let $D$ be the midpoint of arc $BC$ that contains $A$. Define $E$ and $F$ similarly. Let the incircle of $ABC$ touches $BC,CA,AB$ at $K,L,M$ respectively. Prove that $DK,EL,FM$ are concurrent.
2010 Iran MO (3rd Round), 5
suppose that $\mathcal F\subseteq p(X)$ and $|X|=n$. prove that if $|\mathcal F|>\sum_{i=0}^{k-1}\dbinom{n}{i}$ then there exist $Y\subseteq X$ with $|Y|=k$ such that $p(Y)=\mathcal F\cap Y$ that $\mathcal F\cap Y=\{F\cap Y:F\in \mathcal F\}$(20 points)
you can see this problem also here:
COMBINATORIAL PROBLEMS AND EXERCISES-SECOND EDITION-by LASZLO LOVASZ-AMS CHELSEA PUBLISHING- chapter 13- problem 10(c)!!!
2014 Balkan MO, 1
Let $x,y$ and $z$ be positive real numbers such that $xy+yz+xz=3xyz$. Prove that \[ x^2y+y^2z+z^2x \ge 2(x+y+z)-3 \] and determine when equality holds.
[i]UK - David Monk[/i]
2002 China Team Selection Test, 1
In acute triangle $ ABC$, show that:
$ \sin^3{A}\cos^2{(B \minus{} C)} \plus{} \sin^3{B}\cos^2{(C \minus{} A)} \plus{} \sin^3{C}\cos^2{(A \minus{} B)} \leq 3\sin{A} \sin{B} \sin{C}$
and find out when the equality holds.
1998 Romania National Olympiad, 3
A ring $A$ is called Boolean if $x^2 = x$ for every $x \in A$. Prove that:
a) A finite set $A$ with $n \geq 2$ elements can be equipped with the structure of a Boolean ring if and only if $n = 2^k$ for some positive integer $k$.
b) The set of nonnegative integers can be equipped with the structure of a Boolean ring.
2014 Peru IMO TST, 5
$n$ vertices from a regular polygon with $2n$ sides are chosen and coloured red. The other $n$ vertices are coloured blue. Afterwards, the $\binom{n}{2}$ lengths of the segments formed with all pairs of red vertices are ordered in a non-decreasing sequence, and the same procedure is done with the $\binom{n}{2}$ lengths of the segments formed with all pairs of blue vertices. Prove that both sequences are identical.
2022 Saudi Arabia IMO TST, 1
Let $(a_n)$ be the integer sequence which is defined by $a_1= 1$ and
$$ a_{n+1}=a_n^2 + n \cdot a_n \,\, , \,\, \forall n \ge 1.$$
Let $S$ be the set of all primes $p$ such that there exists an index $i$ such that $p|a_i$.
Prove that the set $S$ is an infinite set and it is not equal to the set of all primes.
2015 IFYM, Sozopol, 6
Find all functions $f: \mathbb{R}\rightarrow \mathbb{R}$ such that for $\forall$ $x,y\in \mathbb{R}$ :
$f(x+f(x+y))+xy=yf(x)+f(x)+f(y)+x$.
2006 JHMT, 8
Circles $P$, $Q$, and $R$ are externally tangent to one another. The external tangent of $P$ and $Q$ that does not intersect $R$ intersects $P$ and $Q$ at $P_Q$ and $Q_P$ , respectively. $Q_R$,$R_Q$,$R_P$ , and $P_R$ are defined similarly. If the radius of $Q$ is $4$ and $\overline{Q_PP_Q} \parallel \overline{R_QQ_R}$, compute $R_PP_R$.
1992 IMO Longlists, 29
Show that in the plane there exists a convex polygon of 1992 sides satisfying the following conditions:
[i](i)[/i] its side lengths are $ 1, 2, 3, \ldots, 1992$ in some order;
[i](ii)[/i] the polygon is circumscribable about a circle.
[i]Alternative formulation:[/i] Does there exist a 1992-gon with side lengths $ 1, 2, 3, \ldots, 1992$ circumscribed about a circle? Answer the same question for a 1990-gon.
2016 Hong Kong TST, 3
2016 circles with radius 1 are lying on the plane. Among these 2016 circles, show that one can select a collection $C$ of 27 circles satisfying the following: either every pair of two circles in $C$ intersect or every pair of two circles in $C$ does not intersect.
2019 Taiwan TST Round 1, 5
Let $a_0,a_1,a_2,\dots $ be a sequence of real numbers such that $a_0=0, a_1=1,$ and for every $n\geq 2$ there exists $1 \leq k \leq n$ satisfying \[ a_n=\frac{a_{n-1}+\dots + a_{n-k}}{k}. \]Find the maximum possible value of $a_{2018}-a_{2017}$.
2011 Cuba MO, 2
A cube of dimensions $20 \times 20 \times 20$ is constructed with blocks of $1 \times 2 \times 2$. Prove that there is a line that passes through the cube but not any block.
2017 India Regional Mathematical Olympiad, 2
Show that the equation \(a^3+(a+1)^3+\ldots+(a+6)^3=b^4+(b+1)^4\) has no solutions in integers \(a,b\).
1999 Flanders Math Olympiad, 4
Let $a,b,m,n$ integers greater than 1. If $a^n-1$ and $b^m+1$ are both primes, give as much info as possible on $a,b,m,n$.
2012 China Team Selection Test, 2
Given an integer $k\ge 2$. Prove that there exist $k$ pairwise distinct positive integers $a_1,a_2,\ldots,a_k$ such that for any non-negative integers $b_1,b_2,\ldots,b_k,c_1,c_2,\ldots,c_k$ satisfying $a_1\le b_i\le 2a_i, i=1,2,\ldots,k$ and $\prod_{i=1}^{k}b_i^{c_i}<\prod_{i=1}^{k}b_i$, we have
\[k\prod_{i=1}^{k}b_i^{c_i}<\prod_{i=1}^{k}b_i.\]
1969 IMO Longlists, 57
Given triangle $ ABC $ with points $ M $ and $ N $ are in the sides $ AB $ and $ AC $ respectively.
If $ \dfrac{BM}{MA} +\dfrac{CN}{NA} = 1 $ , then prove that the centroid of $ ABC $ lies on $ MN $ .
2022/2023 Tournament of Towns, P5
On the sides of a regular nonagon $ABCDEFGHI$, triangles $XAB, YBC, ZCD$ and $TDE$ are constructed outside the nonagon. The angles at $X, Y, Z, T$ in these triangles are each $20^\circ$. The angles $XAB, YBC, ZCD$ and $TDE$ are such that (except for the first angle) each angle is $20^\circ$ greater than the one listed before it. Prove that the points $X, Y , Z, T$ lie on the same circle.