Found problems: 85335
2013 239 Open Mathematical Olympiad, 8
The product of the positive numbers $a, b, c, d$ and $e$ is equal to $1$. Prove that
$$ \frac{a^2}{b^2}+\frac{b^2}{c^2}+\frac{c^2}{d^2}+\frac{d^2}{e^2}+\frac{e^2}{a^2} \geq a+b+c+d+e .$$
2018 239 Open Mathematical Olympiad, 8-9.3
Is it possible to divide all non-empty subsets of a set of 10 elements into triples so that in each triple, two of the subsets do not intersect and in their union give the third?
[i]Proposed by Vladislav Frank[/i]
2006 Miklós Schweitzer, 9
Does the circle T = R / Z have a self-homeomorphism $\phi$ that is singular (that is, its derivative is almost everywhere 0), but the mapping $f:T \to T$ , $f(x) = \phi^{-1} (2\phi(x))$ is absolutely continuous?
2001 Korea - Final Round, 3
Let $x_1,x_2, \cdots,x_n$ and $y_1,y_2, \cdots ,y_n$ be arbitrary real numbers satisfying $x_1^2+x_2^2+\cdots+x_n^2=y_1^2+y_2^2+\cdots+y_n^2=1$. Prove that
\[(x_1y_2-x_2y_1)^2 \le 2\left|1-\sum_{k=1}^n x_ky_k\right|\]
and find all cases of equality.
2011 Mongolia Team Selection Test, 3
Let $n$ and $d$ be positive integers satisfying $d<\dfrac{n}{2}$. There are $n$ boys and $n$ girls in a school. Each boy has at most $d$ girlfriends and each girl has at most $d$ boyfriends. Prove that one can introduce some of them to make each boy have exactly $2d$ girlfriends and each girl have exactly $2d$ boyfriends. (I think we assume if a girl has a boyfriend, she is his girlfriend as well and vice versa)
(proposed by B. Batbaysgalan, folklore).
1995 All-Russian Olympiad Regional Round, 9.8
Can the numbers $1,2,...,121$ be written in the cells of an $11\times 11$ board in such a way that any two consecutive numbers are in adjacent cells (sharing a side), and all perfect squares are in the same column?
Kyiv City MO Seniors 2003+ geometry, 2022.10.2
Diagonals of a cyclic quadrilateral $ABCD$ intersect at point $P$. The circumscribed circles of triangles $APD$ and $BPC$ intersect the line $AB$ at points $E, F$ correspondingly. $Q$ and $R$ are the projections of $P$ onto the lines $FC, DE$ correspondingly. Show that $AB \parallel QR$.
[i](Proposed by Mykhailo Shtandenko)[/i]
2019 Hanoi Open Mathematics Competitions, 14
Let $a, b, c$ be nonnegative real numbers satisfying $a + b + c =3$.
a) If $c > \frac32$, prove that $3(ab + bc + ca) - 2abc < 7$.
b) Find the greatest possible value of $M =3(ab + bc + ca) - 2abc $.
2015 Estonia Team Selection Test, 3
Let $q$ be a fixed positive rational number. Call number $x$ [i]charismatic [/i] if there exist a positive integer $n$ and integers $a_1, a_2, . . . , a_n$ such that $x = (q + 1)^{a_1} \cdot (q + 2)^{a_2} ...(q + n)^{a_n}$.
a) Prove that $q$ can be chosen in such a way that every positive rational number turns out to be charismatic.
b) Is it true for every $q$ that, for every charismatic number $x$, the number $x + 1$ is charismatic, too?
2013 Iran Team Selection Test, 11
Let $a,b,c$ be sides of a triangle such that $a\geq b \geq c$. prove that:
$\sqrt{a(a+b-\sqrt{ab})}+\sqrt{b(a+c-\sqrt{ac})}+\sqrt{c(b+c-\sqrt{bc})}\geq a+b+c$
2010 Kosovo National Mathematical Olympiad, 2
Let $a_1,a_2,...,a_n$ be an arithmetic progression of positive real numbers. Prove that
$\tfrac {1}{\sqrt a_1+\sqrt a_2}+\tfrac {1}{\sqrt a_2+\sqrt a_3}+...+\tfrac {1}{\sqrt a_{n-1}+\sqrt a_n}=\tfrac{n-1}{\sqrt {a_1}+\sqrt{a_n}}$.
1999 French Mathematical Olympiad, Problem 4
On a table are given $1999$ red candies and $6661$ yellow candies. The candies are indistinguishable due to the same packing. A gourmet applies the following procedure as long as it is possible:
(i) He picks any of the remaining candies, notes its color, eats it and goes to (ii).
(ii) He picks any of the remaining candies, and notes its color: if it is the same as the color of the last eaten candy, eats it and goes to (ii); otherwise returns it upon repacking and goes to (i).
Prove that all the candies will be eaten and find the probability that the last eaten candy will be red.
2008 Bosnia and Herzegovina Junior BMO TST, 1
Let $ a,b,c$ be real positive numbers such that absolute difference between any two of them is less than $ 2$. Prove that: $ a \plus{} b \plus{} c < \sqrt {ab \plus{} 1} \plus{} \sqrt {ac \plus{} 1} \plus{} \sqrt {bc \plus{} 1}$
2022 DIME, 7
Richard has an infinite row of empty boxes labeled $1, 2, 3, \ldots$ and an infinite supply of balls. Each minute, Richard finds the smallest positive integer $k$ such that box $k$ is empty. Then, Richard puts a ball into box $k$, and if $k \geq 3$, he removes one ball from each of boxes $1,2,\ldots,k-2$. Find the smallest positive integer $n$ such that after $n$ minutes, both boxes $9$ and $10$ have at least one ball in them.
[i]Proposed by [b]vvluo[/b] & [b]richy[/b][/i]
2016 Belarus Team Selection Test, 3
Point $A,B$ are marked on the right branch of the hyperbola $y=\frac{1}{x},x>0$. The straight line $l$ passing through the origin $O$ is perpendicular to $AB$ and meets $AB$ and given branch of the hyperbola at points $D$ and $C$ respectively. The circle through $A,B,C$ meets $l$ at $F$.
Find $OD:CF$
2024 Francophone Mathematical Olympiad, 1
Find the largest integer $k$ with the following property: Whenever real numbers $x_1,x_2,\dots,x_{2024}$ satisfy
\[x_1^2=(x_1+x_2)^2=\dots=(x_1+x_2+\dots+x_{2024})^2,\]
at least $k$ of them are equal.
1991 Baltic Way, 1
Find the smallest positive integer $n$ having the property that for any $n$ distinct integers $a_1, a_2, \dots , a_n$ the product of all differences $a_i-a_j$ $(i < j)$ is divisible by $1991$.
2004 Chile National Olympiad, 2
Every point on a line is painted either red or blue. Prove that there always exist three points $A,B,C$ that are painted the same color and are such that the point $B$ is the midpoint of the segment $AC$.
2010 CHMMC Fall, 3
Talithia throws a party on the fifth Saturday of every month that has five Saturdays. That is, if a month has five Saturdays, Talithia has a party on the fifth Saturday of that month, and if a month has four Saturdays, then Talithia does not have a party that month. Given that January $1$, $2010$ was a Friday, compute the number of parties Talithia will have in $2010$.
2019 District Olympiad, 3
Consider the rectangular parallelepiped $ABCDA'B'C'D' $ as such the measure of the dihedral angle formed by the planes $(A'BD)$ and $(C'BD)$ is $90^o$ and the measure of the dihedral angle formed by the planes $(AB'C)$ and $(D'B'C)$ is $60^o$. Determine and measure the dihedral angle formed by the planes $(BC'D)$ and $(A'C'D)$.
2023 Switzerland - Final Round, 7
In the acute-angled triangle $ABC$, the point $F$ is the foot of the altitude from $A$, and $P$ is a point on the segment $AF$. The lines through $P$ parallel to $AC$ and $AB$ meet $BC$ at $D$ and $E$, respectively. Points $X \ne A$ and $Y \ne A$ lie on the circles $ABD$ and $ACE$, respectively, such that $DA = DX$ and $EA = EY$.
Prove that $B, C, X,$ and $Y$ are concyclic.
2014 Vietnam National Olympiad, 3
Find all sets of not necessary distinct 2014 rationals such that:if we remove an arbitrary number in the set, we can divide remaining 2013 numbers into three sets such that each set has exactly 671 elements and the product of all elements in each set are the same.
2010 Today's Calculation Of Integral, 622
For $0<k<2$, consider two curves $C_1: y=\sin 2x\ (0\leq x\leq \pi),\ C_2: y=k\cos x\ (0\leqq x\leqq \pi).$
Denote by $S(k)$ the sum of the areas of four parts enclosed by $C_1,\ C_2$ and two lines $x=0,\ x=\pi$.
Find the minimum value of $S(k).$
[i]2010 Nagoya Institute of Technology entrance exam[/i]
2014 IPhOO, 3
Which of the following derived units is equivalent to units of velocity?
$ \textbf {(A) } \dfrac {\text {W}}{\text {N}} \qquad \textbf {(B) } \dfrac {\text {N}}{\text {W}} \qquad \textbf {(C) } \dfrac {\text {W}}{\text {N}^2} \qquad \textbf {(D) } \dfrac {\text {W}^2}{\text {N}} \qquad \textbf {(E) } \dfrac {\text {N}^2}{\text {W}^2} $
[i]Problem proposed by Ahaan Rungta[/i]
2020 Brazil Team Selection Test, 1
Determine if there is a positive integer $n$ such that for any $n$ consecutive positive integers, there is [b]one[/b] of them(denote $c$) such that $c$ can be written as sum of consecutive integers(not necessarily all positive) of at most $2020$ distinct ways.