[b]a)[/b] Show that there is no function $ f:\mathbb{R}\longrightarrow\mathbb{R} $ having the property that $$ f(f(x))=\left\{ \begin{matrix} \sqrt{2007} ,& \quad x\in\mathbb{Q} \\ 2007, & \quad x\not\in \mathbb{Q} \end{matrix} \right. , $$ for any real number $ x. $ [b]b)[/b] Prove that there is an infinite number of functions $ g:\mathbb{R}\longrightarrow\mathbb{R} $ having the property that $$ g(g(x))=\left\{ \begin{matrix} 2007 ,& \quad x\in\mathbb{Q} \\ \sqrt{2007}, & \quad x\not\in \mathbb{Q} \end{matrix} \right. , $$ for any real number $ x. $

Let $ABCD$ be a convex quadrilateral with $\angle ABC>90$, $CDA>90$ and $\angle DAB=\angle BCD$. Denote by $E$ and $F$ the reflections of $A$ in lines $BC$ and $CD$, respectively. Suppose that the segments $AE$ and $AF$ meet the line $BD$ at $K$ and $L$, respectively. Prove that the circumcircles of triangles $BEK$ and $DFL$ are tangent to each other. $\emph{Slovakia}$

(a) Prove that $\sqrt{n+1}-\sqrt n<\frac1{2\sqrt n}<\sqrt n-\sqrt{n-1}$ for all $n\in\mathbb N$. (b) Prove that the integer part of the sum $1+\frac1{\sqrt2}+\frac1{\sqrt3}+\ldots+\frac1{\sqrt{m^2}}$, where $m\in\mathbb N$, is either $2m-2$ or $2m-1$.

The sum of the distances from one vertex of a square with sides of length two to the midpoints of each of the sides of the square is $\textbf{(A) }2\sqrt{5}\qquad\textbf{(B) }2+\sqrt{3}\qquad\textbf{(C) }2+2\sqrt{3}\qquad\textbf{(D) }2+\sqrt{5}\qquad \textbf{(E) }2+2\sqrt{5}$

Numbers $a, b, c$ are the length of the medians of some triangle. If $ab + bc + ac = 1$ prove that a) $a^2b + b^2c + c^2a > \frac13$ b) $a^2b + b^2c + c^2a > \frac12$ (I. Bliznets)

In the diagram below, rectangle $ABCD$ has $AB = 5$ and $AD = 12$. Also, $E$ is a point in the same plane outside $ABCD$ such that the perpendicular distances from $E$ to the lines $AB$ and $AD$ are $12$ and $1$, respectively, and $\triangle ABE$ is acute. There exists a line passing through $E$ which splits $ABCD$ into two figures of equal area. Suppose that this line intersects $\overline{AB}$ at a point $F$ and $\overline{CD}$ at a point $G$. Find $FG^2$. [asy] size(6.5cm); pair B=(0,0), C=(12,0), D=(12,5), A=(0,5); pair E=(-12,4); draw(A--E--B--C--D--cycle); draw(A--B); dot("$A$", A, NW); dot("$B$", B, SW); dot("$C$", C, SE); dot("$D$", D, NE); dot("$E$", E, W); [/asy] [i]Proposed by ApraTrip[/i]

All members of the senate were firstly divided into $S$ senate commissions . According to the rules, no commission has less that $5$ senators and every two commissions have different number of senators. After the first session the commissions were closed and new commissions were opened. Some of the senators now are not a part of any commission. It resulted also that every two senators that were in the same commission in the first session , are not any more in the same commission. [b](a)[/b]Prove that at least $4S+10$ senators were left outside the commissions. [b](b)[/b]Prove that this number is achievable. Albanian National Mathematical Olympiad 2010---12 GRADE Question 5.

Let $ABC$ and $A'B'C'$ be any two coplanar triangles. Let $L$ be a point such that $AL || BC, A'L || B'C'$ , and $M,N$ similarly defined. The line $BC$ meets $B'C'$ at $P$, and similarly defined are $Q$ and $R$. Prove that $PL, QM, RN$ are concurrent.

Angle bisectors $AD$ and $CE$ of triangle $ABC$ intersect at point $O$. A line symmetrical to $ AB$ with respect to $CE$ intersects the line symmetric $BC$ with respect to $AD$, at point $K$. Prove that $KO \perp AC$.

Let $n>3$ be a positive integer. Prove that $n$ is prime if and only if there exists a positive integer $\alpha$ such that $n!=n(n-1)(\alpha n+1)$.

Consider the graph $G$ with $100$ vertices, and the minimum odd cycle goes through $13$ vertices. Prove that the vertices of the graph can be colored in $6$ colors in a way that no two adjacent vertices have the same color.

Find all positive integers $(a,b,c)$ such that $$ab-c,\quad bc-a,\quad ca-b$$ are all powers of $2$. [i]Proposed by Serbia[/i]

Let $\mathbb N_0$ be the set of non-negative integers. There is a triple $(f,a,b)$, where $f$ is a function from $\mathbb N_0$ to $\mathbb N_0$ and $a,b\in\mathbb N_0$ that satisfies the following conditions: [list] [*]$f(1)=2$ [*]$f(a)+f(b)\leq 2\sqrt{f(a)}$ [*]For all $n>0$, we have $f(n)=f(n-1)f(b)+2n-f(b)$ [/list] Find the sum of all possible values of $f(b+100)$.

Given positive integers $a,b,$ find the least positive integer $m$ such that among any $m$ distinct integers in the interval $[-a,b]$ there are three pair-wise distinct numbers that their sum is zero. [i]Proposed by Marian Tetiva, Romania[/i]

For any measurable set $H \subset R$ , we define the sequence $a_n(H)$ by the formula: $$a_n(H) = \lambda \bigg([0,1] \setminus \bigcup_{k = n}^{2n} (H + \log_2 k) \bigg)$$ where $\lambda$ denotes the Lebesgue measure and $\log_2$ denotes the binary logarithm. Prove that there is a measurable, 1-periodic, positive measure set $H \subset R$ , such that the sequence $a_n( H )$ does not belong to any space $l_p$ ($1 \leq p < \infty$). [hide=not sure about this part]For what numbers $1 \leq p <\infty$ is it true that whenever H is 1-periodic, positive measure, the sequence $a_n( H )$ belongs to the space $l_p$?[/hide]

Five guys each have a positive integer (the integers are not necessarily distinct). The greatest common divisor of any two guys' numbers is always more than $1$, but the greatest common divisor of all the numbers is $1$. What is the minimum possible value of the product of the numbers?

The sides of a convex pentagon are extended on both sides to form five triangles. If these triangles are congruent to one another, does it follow that the pentagon is regular?

For positive integers $x, y, $ $r_x(y)$ to represent the smallest positive integer $ r $ such that $ r \equiv y(\text{mod x})$ .For any positive integers $a, b, n ,$ Prove that $$\sum_{i=1}^{n} r_b(a i)\leq \frac{n(a+b)}{2}$$

Christian Reiher and Reid Barton want to open a security box, they already managed to discover the algorithm to generate the key codes and they obtained the following information: $i)$ In the screen of the box will appear a sequence of $n+1$ numbers, $C_0 = (a_{0,1},a_{0,2},...,a_{0,n+1})$ $ii)$ If the code $K = (k_1,k_2,...,k_n)$ opens the security box then the following must happen: a) A sequence $C_i = (a_{i,1},a_{i,2},...,a_{i,n+1})$ will be asigned to each $k_i$ defined as follows: $a_{i,1} = 1$ and $a_{i,j} = a_{i-1,j}-k_ia_{i,j-1}$, for $i,j \ge 1$ b) The sequence $(C_n)$ asigned to $k_n$ satisfies that $S_n = \sum_{i=1}^{n+1}|a_i|$ has its least possible value, considering all possible sequences $K$. The sequence $C_0$ that appears in the screen is the following: $a_{0,1} = 1$ and $a_0,i$ is the sum of the products of the elements of each of the subsets with $i-1$ elements of the set $A =$ {$1,2,3,...,n$}, $i\ge 2$, such that $a_{0, n+1} = n!$ Find a sequence $K = (k_1,k_2,...,k_n)$ that satisfies the conditions of the problem and show that there exists at least $n!$ of them.

Compute $$\sum^{\infty}_{n=1} \left( \frac{1}{n^2 + 3n} - \frac{1}{n^2 + 3n + 2}\right)$$

Prove that any triangle can be divided into $22$ triangles, each of which has an angle of $22^o$, and another $23$ triangles, each of which has an angle of $23^o$.

Let $n \geq 2$ be a positive integer. Suppose there exist non-negative integers ${n_1},{n_2},\ldots,{n_k}$ such that $2^n - 1 \mid \sum_{i = 1}^k {{2^{{n_i}}}}$. Prove that $k \ge n$.

Find all pairs of functions $ f, g: \mathbb R \to \mathbb R$ such that [list] (i) if $ x < y$, then $ f(x) < f(y)$; (ii) $ f(xy) \equal{} g(y)f(x) \plus{} f(y)$ for all $ x, y \in \mathbb R$. [/list]

Let $ m_1,m_2,m_3,\dots,m_n$ be a rearrangement of the numbers $ 1,2,\dots,n$. Suppose that $ n$ is odd. Prove that the product \[ \left(m_1\minus{}1\right)\left(m_2\minus{}2\right)\cdots \left(m_n\minus{}n\right)\] is an even integer.

$ K$ takes $ 30$ minutes less time than $ M$ to travel a distance of $ 30$ miles. $ K$ travels $ \frac {1}{3}$ mile per hour faster than $ M$. If $ x$ is $ K$'s rate of speed in miles per hours, then $ K$'s time for the distance is: $ \textbf{(A)}\ \dfrac{x \plus{} \frac {1}{3}}{30} \qquad\textbf{(B)}\ \dfrac{x \minus{} \frac {1}{3}}{30} \qquad\textbf{(C)}\ \dfrac{30}{x \plus{} \frac {1}{3}} \qquad\textbf{(D)}\ \frac {30}{x} \qquad\textbf{(E)}\ \frac {x}{30}$