Prove that circle l(0,2) with equation $x^2+y^2=4$ contains infinite points with rational coordinates

Given circles $ \omega_1$ and $ \omega_2$ intersecting at points $ X$ and $ Y$, let $ \ell_1$ be a line through the center of $ \omega_1$ intersecting $ \omega_2$ at points $ P$ and $ Q$ and let $ \ell_2$ be a line through the center of $ \omega_2$ intersecting $ \omega_1$ at points $ R$ and $ S$. Prove that if $ P, Q, R$ and $ S$ lie on a circle then the center of this circle lies on line $ XY$.

The equality $ \frac{1}{x\minus{}1}\equal{}\frac{2}{x\minus{}2}$ is satisfied by: $ \textbf{(A)}\ \text{no real values of }x \qquad \textbf{(B)}\ \text{either }x\equal{}1 \text{ or }x\equal{}2 \qquad \textbf{(C)}\ \text{only }x\equal{}1 \\ \textbf{(D)}\ \text{only }x\equal{}2 \qquad \textbf{(E)}\ \text{only }x\equal{}0$

Let $ z \in \mathbb{C}$ such that for all $ k \in \overline{1, 3}$, $ |z^k \plus{} 1| \le 1$. Prove that $ z \equal{} 0$.

Laura and Daniel play with quadratic polynomials. First Laura says a nonzero real number $r$. Then Daniel says a nonzero real number $s$, and then again Laura says another nonzero real number $t$. Finally. Daniel writes the polynomial $P(x) = ax^2 + bx + c$ where $a,b$, and $c$ are $r,s$, and $t$ in some order Daniel chooses. Laura wins if the equation $P(x) = 0$ has two different real solutions, and Daniel wins otherwise. Determine who has a winning strategy and describe that strategy.

Let $E$ be a point outside of square $ABCD$. If the distance of $E$ to $AC$ is $6$, to $BD$ is $17$, and to the nearest vertex of the square is $10$, what is the area of the square? $ \textbf{(A)}\ 200 \qquad\textbf{(B)}\ 196 \qquad\textbf{(C)}\ 169 \qquad\textbf{(D)}\ 162 \qquad\textbf{(E)}\ 144 $

Show that no one $n$-th root of a rational (for $n$ a positive integer) can be a root of the polynomial $x^5 - x^4 - 4x^3 + 4x^2 + 2$.

The acuteangled triangle $ABC$ with circumcenter $O$ is given. The midpoints of the sides $BC, CA$ and $AB$ are $D, E$ and $F$ respectively. An arbitrary point $M$ on the side $BC$, different of $D$, is choosen. The straight lines $AM$ and $EF$ intersects at the point $N$ and the straight line $ON$ cut again the circumscribed circle of the triangle $ODM$ at the point $P$. Prove that the reflection of the point $M$ with respect to the midpoint of the segment $DP$ belongs on the nine points circle of the triangle $ABC$.

Let $n$ be a positive integer. Given is a subset $A$ of $\{0,1,...,5^n\}$ with $4n+2$ elements. Prove that there exist three elements $a<b<c$ from $A$ such that $c+2a>3b$. [i]Proposed by Dominik Burek and Tomasz Ciesla, Poland[/i]

Let $q$ be a positive rational number. Two ants are initially at the same point $X$ in the plane. In the $n$-th minute $(n = 1,2,...)$ each of them chooses whether to walk due north, east, south or west and then walks the distance of $q^n$ metres. After a whole number of minutes, they are at the same point in the plane (not necessarily $X$), but have not taken exactly the same route within that time. Determine all possible values of $q$. Proposed by Jeremy King, UK

A kite is inscribed in a circle with center $O$ and radius $60$. The diagonals of the kite meet at a point $P$, and $OP$ is an integer. The minimum possible area of the kite can be expressed in the form $a\sqrt{b}$, where $a$ and $b$ are positive integers and $b$ is squarefree. Find $a+b$.

Divide the unit square into $9$ equal squares by means of two pairs of lines parallel to the sides (see figure). Now remove the central square. Treat the remaining $8$ squares the same way, and repeat the process $n$ times. (a) How many squares of side length $1/3^n$ remain? (b) What is the sum of the areas of the removed squares as $n$ becomes infinite? [center][img]https://cdn.artofproblemsolving.com/attachments/7/d/3e6e68559919583c24d4457f946bc4cef3922f.png[/img][/center]

Let $I$ be the center of the circle inscribed in triangle $ABC$. The inscribed circle is tangent to side $BC$ at point $K$. Let $X$ and $Y$ be points on segments $BI$ and $CI$ respectively, such that $KX \perp AB $ and $KY\perp AC$. The circumscribed circle around triangle $XYK$ intersects line $BC$ at point $D$. Prove that $AD \perp BC$. (Matthew Kurskyi)

An isosceles triangle $ ABC$ with $ AB \equal{} AC$ is given on the plane. A variable circle $ (O)$ with center $ O$ on the line $ BC$ passes through $ A$ and does not touch either of the lines $ AB$ and $ AC$. Let $ M$ and $ N$ be the second points of intersection of $ (O)$ with lines $ AB$ and $ AC$, respectively. Find the locus of the orthocenter of triangle $ AMN$.

Prove that the system of equations $\begin{cases} x_1 - x_2 = a \\ x_3 - x_4 = b \\ x_1 + x_2 + x_3 + x_4 = 1\end{cases}$ has at least one solution in positive numbers ($x_1 ,x_2 ,x_3 ,x_4>0$) if and only if $|a| + |b| < 1$.

Given that $x_{n+2}=\dfrac{20x_{n+1}}{14x_n}$, $x_0=25$, $x_1=11$, it follows that $\sum_{n=0}^\infty\dfrac{x_{3n}}{2^n}=\dfrac pq$ for some positive integers $p$, $q$ with $GCD(p,q)=1$. Find $p+q$.

[color=darkred]A circle $ \Omega_1$ is tangent outwardly to the circle $ \Omega_2$ of bigger radius. Line $ t_1$ is tangent at points $ A$ and $ D$ to the circles $ \Omega_1$ and $ \Omega_2$ respectively. Line $ t_2$, parallel to $ t_1$, is tangent to the circle $ \Omega_1$ and cuts $ \Omega_2$ at points $ E$ and $ F$. Point $ C$ belongs to the circle $ \Omega_2$ such that $ D$ and $ C$ are separated by the line $ EF$. Denote $ B$ the intersection of $ EF$ and $ CD$. Prove that circumcircle of $ ABC$ is tangent to the line $ AD$.[/color]

There are $9$ vertical columns in a row. In some places, horizontal sticks are inserted between adjacent columns, no two are at the same height. The beetle crawls from the bottom up; when he meets the wand, he crawls over it to the next column and continues to crawl up. It is known that if a beetle starts at the bottom of the first (leftmost) column, then it will end its journey on the ninth (rightmost) column. Is it always possible to remove one stick so that the beetle ends up at the top of the fifth column? (For example, if the sticks are arranged as in picture, the beetle will crawl along a solid line. If you remove the third one A stick in the path of the beetle, then it will crawl along the dotted line.) [i] Proposed by G. Karavaev[/i]

In a right-angled triangle, the sum $a + b$ of the sides enclosing the right angle equals $24$ while the length of the altitude $h_c$ on the hypotenuse $c$ is $7$. Determine the length of the hypotenuse.

A circle is inscribed in triangle $ABC$. $M$ and $N$ are the points of its tangency with the sides $BC$ and $CA$, respectively. The segment $AM$ intersects $BN$ at point $P$ and the inscribed circle at point $Q$. It is known that $MP = a$, $PQ = b$. Find $AQ$.

Let $a_1,a_2,\ldots, a_{2013}$ be a permutation of the numbers from $1$ to $2013$. Let $A_n = \frac{a_1 + a_2 + \cdots + a_n} {n}$ for $n = 1,2,\ldots, 2013$. If the smallest possible difference between the largest and smallest values of $A_1,A_2,\ldots, A_{2013}$ is $\frac{m}{n}$, where $m$ and $n$ are relatively prime positive integers, find $m+n$. [i]Ray Li[/i]

Two sequences $(a_n)$ and $(b_n)$ satisfy that $a_0=1,a_1=4,a_2=49$, and $\begin{cases} a_{n+1}=7a_n+6b_n-3\\ b_{n+1}=8a_n+7b_n-4\\ \end{cases}$ for $n=0,1,2,\cdots,$. Prove that $a_n$ is a perfect square for $n=0,1,2,\cdots,$.

When we cut a rope into two pieces, we say that the cut is special if both pieces have different lengths. We cut a chord of length $2008$ into two pieces with integer lengths and we write those lengths on the board. Afterwards, we cut one of the pieces into two new pieces with integer lengths and we write those lengths on the board. This process ends until all pieces have length $1$. $a)$ Find the minimum possible number of special cuts. $b)$ Prove that, for all processes that have the minimum possible number of special cuts, the number of different integers on the board is always the same.

Let $A,B$ be $n\times n$ matrices, $n\geq 2$, and $B^2=B$. Prove that: $$\text{rank}\,(AB-BA)\leq \text{rank}\,(AB+BA)$$

(a)The game of "super- chess" is played on a $30 \times 30$ board and involves $20$ different pieces. Each piece moves according to its own rules , but cannot move from any square to more than $20$ other squares . A piece "captures" another piece which is on a square to which it has moved. A permitted move (e.g. $m$ squares forward and $n$ squares to the right) does not depend on the piece 's starting square . Prove that (i) A piece cannot cap ture a piece on a given square from more than $20$ starting squares. (ii) It is possible to arrange all $20$ pieces on the board in such a way that not one of them can capture any of the others in one move. (b) The game of "super-chess" is played on a $100 \times 100$ board and involves $20$ different pieces. Each piece moves according to its own rules , but cannot move from any square to more than $20$ other squares. A piece "captures" another piece which is on a square to which it has moved. It is possible that a permitted move (e.g. $m$ squares forward and $n$ squares to the right) may vary, depending on the piece's position . Prove that one can arrange all $20$ pieces on the board in such a way that not one of them can capture any of the others in one move. ( A . K . Tolpygo, Kiev) PS. (a) for Juniors , (b) for Seniors