Found problems: 85335
1998 National Olympiad First Round, 5
$ ABCD$ is a cyclic quadrilateral. If $ \angle B \equal{} \angle D$, $ AC\bigcap BD \equal{} \left\{E\right\}$, $ \angle BCD \equal{} 150{}^\circ$, $ \left|BE\right| \equal{} x$, $ \left|AC\right| \equal{} z$, then find $ \left|ED\right|$ in terms of $ x$ and $ z$.
$\textbf{(A)}\ \frac {z \minus{} x}{\sqrt {3} } \qquad\textbf{(B)}\ \frac {z \minus{} 2x}{3} \qquad\textbf{(C)}\ \frac {z \plus{} x}{\sqrt {3} } \qquad\textbf{(D)}\ \frac {z \minus{} 2x}{2} \qquad\textbf{(E)}\ \frac {2z \minus{} 3x}{2}$
2007 Argentina National Olympiad, 4
$10$ real numbers are given $a_1,a_2,\ldots ,a_{10} $, and the $45$ sums of two of these numbers are formed $a_i+a_j $, $1\leq i<j\leq 10$ . It is known that not all these sums are integers. Determine the minimum value of $k$ such that it is possible that among the $45$ sums there are $k$ that are not integers and $45-k$ that are integers.
2007 ITest, 35
Find the greatest natural number possessing the property that each of its digits except the first and last one is less than the arithmetic mean of the two neighboring digits.
2006 All-Russian Olympiad Regional Round, 8.3
Four drivers took part in the round-robin racing. Their cars started simultaneously from one point and moved at constant speeds. It is known that after the start of the race, for any three cars there was a moment when they met. Prove that after the start of the race there will be a moment when all 4 cars meet. (We consider races to be infinitely long in time.)
1998 Belarus Team Selection Test, 1
Let $O$ be a point inside an acute angle with the vertex $A$ and $H, N$ be the feet of the perpendiculars drawn from $O$ onto the sides of the angle. Let point $B$ belong to the bisector of the angle, $K$ be the foot of the perpendicular from $B$ onto either side of the angle. Denote by $P,F$ the midpoints of the segments $AK,HN$ respectively. Known that $ON + OH = BK$, prove that $PF$ is perpendicular to $AB$.
Ya. Konstantinovski
2011 Princeton University Math Competition, A4
Suppose the polynomial $x^3 - x^2 + bx + c$ has real roots $a, b, c$. What is the square of the minimum value of $abc$?
1979 Chisinau City MO, 180
It is known that for $0\le x \le 1$ the square trinomial $f (x)$ satisfies the condition $|f(x) | \le 1$. Show that $| f '(0) | \le 8.$
2022 AMC 12/AHSME, 4
For how many values of the constant $k$ will the polynomial $x^{2}+kx+36$ have two distinct integer roots?
$\textbf{(A) }6 \qquad \textbf{(B) }8 \qquad \textbf{(C) }9 \qquad \textbf{(D) }14 \qquad \textbf{(E) }16$
2004 AMC 12/AHSME, 15
Brenda and Sally run in opposite directions on a circular track, starting at diametrically opposite points. They first meet after Brenda has run $ 100$ meters. They next meet after Sally has run $ 150$ meters past their first meeting point. Each girl runs at a constant speed. What is the length of the track in meters?
$ \textbf{(A)}\ 250 \qquad \textbf{(B)}\ 300 \qquad \textbf{(C)}\ 350 \qquad \textbf{(D)}\ 400\qquad \textbf{(E)}\ 500$
2019 LIMIT Category C, Problem 1
Which of the following are true?
$\textbf{(A)}~\forall A\in M_n(\mathbb R),A^t=X^{-1}AX\text{ for some }X\in M_n(\mathbb R)$
$\textbf{(B)}~\forall A\in M_n(\mathbb R),I+AA^t\text{ is invertible}$
$\textbf{(C)}~\operatorname{tr}(AB)=\operatorname{tr}(BA),\forall A,B\in M_n(\mathbb R)\text{ but }\exists A,B,C\text{ such that }\operatorname{tr}(ABC)\ne\operatorname{tr}(BAC)$
$\textbf{(D)}~\text{None of the above}$
2022 MOAA, 5
Find the smallest positive integer that is equal to the sum of the product of its digits and the sum of its digits.
2013 International Zhautykov Olympiad, 2
Find all odd positive integers $n>1$ such that there is a permutation $a_1, a_2, a_3, \ldots, a_n$ of the numbers $1, 2,3, \ldots, n$ where $n$ divides one of the numbers $a_k^2 - a_{k+1} - 1$ and $a_k^2 - a_{k+1} + 1$ for each $k$, $1 \leq k \leq n$ (we assume $a_{n+1}=a_1$).
2020 Indonesia MO, 7
Determine all real-coefficient polynomials $P(x)$ such that
\[ P(\lfloor x \rfloor) = \lfloor P(x) \rfloor \]for every real numbers $x$.
2021 Sharygin Geometry Olympiad, 12
Suppose we have ten coins with radii $1, 2, 3, \ldots , 10$ cm. We can put two of them on the table in such a way that they touch each other, after that we can add the coins in such a way that each new coin touches at least two of previous ones. The new coin cannot cover a previous one. Can we put several coins in such a way that the centers of some three coins are collinear?
2024 Euler Olympiad, Round 1, 1
Using each of the ten digits exactly once, construct two five-digit numbers such that their difference is minimized. Determine this minimal difference.
[i]Proposed by Giorgi Arabidze, Georgia [/i]
2010 Romania Team Selection Test, 1
Given a positive integer number $n$, determine the minimum of
\[\max \left\{\dfrac{x_1}{1 + x_1},\, \dfrac{x_2}{1 + x_1 + x_2},\, \cdots,\, \dfrac{x_n}{1 + x_1 + x_2 + \cdots + x_n}\right\},\]
as $x_1, x_2, \ldots, x_n$ run through all non-negative real numbers which add up to $1$.
[i]Kvant Magazine[/i]
1996 Argentina National Olympiad, 1
$100$ numbers were written around a circle. The sum of the $100$ numbers is equal to $100$ and the sum of six consecutive numbers is always less than or equal to $6$. The first number is $6$. Find all the numbers.
1955 AMC 12/AHSME, 34
A $ 6$-inch and $ 18$-inch diameter pole are placed together and bound together with wire. The length of the shortest wire that will go around them is:
$ \textbf{(A)}\ 12\sqrt{3}\plus{}16\pi \qquad
\textbf{(B)}\ 12\sqrt{3}\plus{}7\pi \qquad
\textbf{(C)}\ 12\sqrt{3}\plus{}14\pi \\
\textbf{(D)}\ 12\plus{}15\pi \qquad
\textbf{(E)}\ 24\pi$
2000 APMO, 3
Let $ABC$ be a triangle. Let $M$ and $N$ be the points in which the median and the angle bisector, respectively, at $A$ meet the side $BC$. Let $Q$ and $P$ be the points in which the perpendicular at $N$ to $NA$ meets $MA$ and $BA$, respectively. And $O$ the point in which the perpendicular at $P$ to $BA$ meets $AN$ produced.
Prove that $QO$ is perpendicular to $BC$.
2018 China Team Selection Test, 4
Let $k, M$ be positive integers such that $k-1$ is not squarefree. Prove that there exist a positive real $\alpha$, such that $\lfloor \alpha\cdot k^n \rfloor$ and $M$ are coprime for any positive integer $n$.
2000 Flanders Math Olympiad, 4
Solve for $x \in [0,2\pi[$: \[\sin x < \cos x < \tan x < \cot x\]
2019 USAJMO, 5
Let $n$ be a nonnegative integer. Determine the number of ways that one can choose $(n+1)^2$ sets $S_{i,j}\subseteq\{1,2,\ldots,2n\}$, for integers $i,j$ with $0\leq i,j\leq n$, such that:
[list]
[*] for all $0\leq i,j\leq n$, the set $S_{i,j}$ has $i+j$ elements; and
[*] $S_{i,j}\subseteq S_{k,l}$ whenever $0\leq i\leq k\leq n$ and $0\leq j\leq l\leq n$.
[/list]
[i]Proposed by Ricky Liu[/i]
2016 Azerbaijan Balkan MO TST, 4
Find all functions $f:\mathbb{N}\to\mathbb{N}$ such that \[f(f(n))=n+2015\] where $n\in \mathbb{N}.$
1996 Brazil National Olympiad, 2
Does there exist a set of $n > 2, n < \infty$ points in the plane such that no three are collinear and the circumcenter of any three points of the set is also in the set?
2017 Kyiv Mathematical Festival, 1
Several dwarves were lined up in a row, and then they lined up in a row in a different order. Is it possible that exactly one third of the dwarves have two new neighbours and exactly one third of the dwarves have only one new neighbour, if the number of the dwarves is a) 9; b) 12?