Found problems: 85335
2023 May Olympiad, 5
There are $100$ boxes that were labeled with the numbers $00$, $01$, $02$,$…$, $99$ . The numbers $000$, $001$, $002$, $…$, $999$ were written on a thousand cards, one on each card. Placing a card in a box is permitted if the box number can be obtained by removing one of the digits from the card number. For example, it is allowed to place card $037$ in box $07$, but it is not allowed to place the card $156$ in box $65$.Can it happen that after placing all the cards in the boxes, there will be exactly $50$ empty boxes?
If the answer is yes, indicate how the cards are placed in the boxes; If the answer is no, explain why it is impossible
2009 Czech and Slovak Olympiad III A, 1
Knowing that the numbers $p, 3p+2, 5p+4, 7p+6, 9p+8$, and $11p+10$ are all primes, prove that $6p+11$ is a composite number.
2019 Jozsef Wildt International Math Competition, W. 27
Find all continuous functions $f : \mathbb{R} \to \mathbb{R}$ such that$$f(-x)+\int \limits_0^xtf(x-t)dt=x,\ \forall\ x\in \mathbb{R}$$
1997 Brazil Team Selection Test, Problem 3
Let $b$ be a positive integer such that $\gcd(b,6)=1$. Show that there are positive integers $x$ and $y$ such that $\frac1x+\frac1y=\frac3b$ if and only if $b$ is divisible by some prime number of form $6k-1$.
2016 District Olympiad, 2
For any natural number $ n, $ denote $ x_n $ as being the number of natural numbers of $ n $ digits that are divisible by $ 4 $ and formed only with the digits $ 0,1,2 $ or $ 6. $
[b]a)[/b] Calculate $ x_1,x_2,x_3,x_4. $
[b]b)[/b] Find the natural number $ m $ such that
$$ 1+\left\lfloor \frac{x_2}{x_1}\right\rfloor +\left\lfloor \frac{x_3}{x_2}\right\rfloor +\left\lfloor \frac{x_4}{x_3}\right\rfloor +\cdots +\left\lfloor \frac{x_{m+1}}{x_m}\right\rfloor =2016 , $$
where $ \lfloor\rfloor $ is the usual integer part.
2011 Gheorghe Vranceanu, 1
Let $ O $ be the circumcenter of $ ABC. $ The equalities
$$ |OA+2OB|=|OB+2OC|=|OC+2OA| $$
hold. Prove that $ ABC $ is equilateral.
2017 Hanoi Open Mathematics Competitions, 5
Let $a, b, c$ be two-digit, three-digit, and four-digit numbers, respectively. Assume that the sum of all digits of number $a+b$, and the sum of all digits of $b + c$ are all equal to $2$. The largest value of $a + b + c$ is
(A): $1099$ (B): $2099$ (C): $1199$ (D): $2199$ (E): None of the above.
1979 VTRMC, 4
Let $f(x)$ be continuously differentiable on $(0,\infty)$ and suppose $ \lim _ { x \rightarrow \infty } f ^ { \prime } ( x ) = 0 $. Prove that $ \lim _ { x \rightarrow \infty } f ( x ) / x = 0 $.
2019 Iran MO (2nd Round), 6
Consider lattice points of a $6*7$ grid.We start with two points $A,B$.We say two points $X,Y$ connected if one can reflect several times WRT points $A,B$ and reach from $X$ to $Y$.Over all choices of $A,B$ what is the minimum number of connected components?
1976 IMO Shortlist, 10
Determine the greatest number, who is the product of some positive integers, and the sum of these numbers is $1976.$
2001 Baltic Way, 6
The points $A, B, C, D, E$ lie on the circle $c$ in this order and satisfy $AB\parallel EC$ and $AC\parallel ED$. The line tangent to the circle $c$ at $E$ meets the line $AB$ at $P$. The lines $BD$ and $EC$ meet at $Q$. Prove that $|AC|=|PQ|$.
2016 Ukraine Team Selection Test, 11
Let $ABC$ be a triangle with $\angle{C} = 90^{\circ}$, and let $H$ be the foot of the altitude from $C$. A point $D$ is chosen inside the triangle $CBH$ so that $CH$ bisects $AD$. Let $P$ be the intersection point of the lines $BD$ and $CH$. Let $\omega$ be the semicircle with diameter $BD$ that meets the segment $CB$ at an interior point. A line through $P$ is tangent to $\omega$ at $Q$. Prove that the lines $CQ$ and $AD$ meet on $\omega$.
2005 CentroAmerican, 2
Show that the equation $a^{2}b^{2}+b^{2}c^{2}+3b^{2}-c^{2}-a^{2}=2005$ has no integer solutions.
[i]Arnoldo Aguilar, El Salvador[/i]
2011 National Olympiad First Round, 26
The integers $0 \leq a < 2^{2008}$ and $0 \leq b < 8$ satisfy the equivalence $7(a+2^{2008}b) \equiv 1 \pmod{2^{2011}}$. Then $b$ is
$\textbf{(A)}\ 3 \qquad\textbf{(B)}\ 5 \qquad\textbf{(C)}\ 6 \qquad\textbf{(D)}\ 7 \qquad\textbf{(E)}\ \text{None}$
2000 May Olympiad, 2
Given a parallelogram with area $1$ and we will construct lines where this lines connect a vertex with a midpoint of the side no adjacent to this vertex; with the $8$ lines formed we have a octagon inside of the parallelogram. Determine the area of this octagon
2019 Sharygin Geometry Olympiad, 20
Let $O$ be the circumcenter of triangle ABC, $H$ be its orthocenter, and $M$ be the midpoint of $AB$. The line $MH$ meets the line passing through $O$ and parallel to $AB$ at point $K$ lying on the circumcircle of $ABC$. Let $P$ be the projection of $K$ onto $AC$. Prove that $PH \parallel BC$.
1993 All-Russian Olympiad Regional Round, 10.7
Points $ M,N$ are taken on sides $ BC,CD$ respectively of parallelogram $ ABCD$. Let $ E\equal{}BD\cap AM, F\equal{}BD\cap AN$. Diagonal $ BD$ cuts triangle $ AMN$ into two parts. Prove that these two parts have equal area if and only if the point $ K$ given by $ EK\parallel{}AD, FK\parallel{}AB$ lies on segment $ MN$.
2021-IMOC, A9
For a given positive integer $n,$ find
$$\sum_{k=0}^{n} \left(\frac{\binom{n}{k} \cdot (-1)^k}{(n+1-k)^2} - \frac{(-1)^n}{(k+1)(n+1)}\right).$$
2016 Purple Comet Problems, 20
The 24 unshaded squares in the 5 × 5 grid below can be tiled with twelve 1 × 2 tiles. One such tiling is shown. Find the number of ways the grid can be tiled.
[center][img]https://snag.gy/KMoPrF.jpg[/img][/center]
2015 İberoAmerican, 1
The number $125$ can be written as a sum of some pairwise coprime integers larger than $1$. Determine the largest number of terms that the sum may have.
1985 Austrian-Polish Competition, 1
Show that if $a+b+c=0$ then $(\frac{a}{b-c}+\frac{b}{c-a}+\frac{c}{a-b})(\frac{b-c}{a}+\frac{c-a}{b}+\frac{a-b}{c})=9$.
2002 Croatia Team Selection Test, 1
In a certain language there are $n$ letters. A sequence of letters is a word, if there are no two equal letters between two other equal letters. Find the number of words of the maximum length.
2013 Princeton University Math Competition, 1
Prove that \[ \frac{1}{a^2+2} + \frac{1}{b^2+2} + \frac{1}{c^2+2} \le \frac{1}{6ab+c^2} + \frac{1}{6bc+a^2} + \frac{1}{6ca+b^2} \] for all positive real numbers $a$, $b$ and $c$ satisfying $a^2+b^2+c^2=1$.
2002 Belarusian National Olympiad, 7
Several clocks lie on the table. It is known that at some moment the sum of distances between a point $X$ of the table and the ends of their minute hands is not equal to the sum of distances between $X$ and the ends of their hour hands.
Prove that there is a moment when the sum of distances between $X$ and the ends of their minute hands is greater than the sum of distances between $X$ and the ends of their hour hands.
(E. Barabanov, I. Voronovich)
2022 Lusophon Mathematical Olympiad, 5
Tow circumferences of radius $R_1$ and $R_2$ are tangent externally between each other. Besides that, they are both tangent to a semicircle with radius of 1, as shown in the figure. (Diagram is in the attachment)
a) If $A_1$ and $A_2$ are the tangency points of the two circumferences with the diameter of the semicircle, find the length of $\overline{A_1 A_2}$.
b) Prove that $R_{1}+R_{2}=2\sqrt{R_{1}R_{2}}(\sqrt{2}-\sqrt{R_{1}R_{2}})$.