Find the smallest natural number $k$ such that the system of equations $$x+y+z=x^2+y^2+z^2=\dots=x^k+y^k+z^k $$ has only one solution for positive real numbers $x$, $y$ and $z$.

Let $c: Q-\{0\} \to Q-\{0\}$ a function with the following properties (for all $x,y, a, b \in Q-\{0\}$ and $x \ne 1$): a) $c (x, 1- x) = 1$ b) $c (ab,y) = c (a,y)c(b, y)$ c) $c (y,ab) = c (y, a)c(y,b)$ Show that then $c (a,b) c(b,a) = 1 = c(a,-a)$ also holds.

A convex pentagon $ABCDE$ with $DC = DE$ and $\angle DCB = \angle DEA = 90^o$ is given. Let $F$ be a point on the segment $AB$ such that $AF : BF = AE : BC$. Prove that $\angle FCE = \angle ADE$ and $\angle FEC = \angle BDC$.

Consider the following sequence of positive real numbers $\dots<a_{-2}<a_{-1}<a_0<a_1<a_2<\dots$ infinite in both directions. For each positive integer $k$ let $b_k$ be the least integer such that the ratio between the sum of $k$ consecutive terms and the greatest of these $k$ terms is less than or equal to $b_k$(This fact occurs for any sequence of $k$ consecutive numbers). Prove that the sequence $b_1,b_2,b_3,...$ coincides with the sequence $1,2,3,...$ or is eventually constant.

[b]p1.[/b] Cities A, B, C, D and E are located next to each other along the highway at a distance of $5$ km from each other. The bus runs along the highway from city A to city E and back. The bus consumes $20$ liters of gasoline for every $100$ kilometers. In which city will a bus run out of gas if it initially had $150$ liters of gasoline in its tank? [b]p2.[/b] Find the minimum four-digit number whose product of all digits is $729$. Explain your answer. [b]p3.[/b] At the parade, soldiers are lined up in two lines of equal length, and in the first line the distance between adjacent soldiers is $ 20\%$ greater than in the second (there is the same distance between adjacent soldiers in the same line). How many soldiers are in the first rank if there are $85$ soldiers in the second rank? [b]p4.[/b] It is known about three numbers that the sum of any two of them is not less than twice the third number, and the sum of all three is equal to $300$. Find all triplets of such (not necessarily integer) numbers. [b]p5.[/b] The tourist fills two tanks of water using two hoses. $2.9$ liters of water flow out per minute from the first hose, $8.7$ liters from the second. At that moment, when the smaller tank was half full, the tourist swapped the hoses, after which both tanks filled at the same time. What is the capacity of the larger tank if the capacity of the smaller one is $12.5$ liters? [b]p6.[/b] Is it possible to mark 6 points on a plane and connect them with non-intersecting segments (with ends at these points) so that exactly four segments come out of each point? [b]p7.[/b] Petya wrote all the natural numbers from $1$ to $1000$ and circled those that are represented as the difference of the squares of two integers. Among the circled numbers, which numbers are more even or odd? [b]p8.[/b] On a sheet of checkered paper, draw a circle of maximum radius that intersects the grid lines only at the nodes. Explain your answer. [b]p9.[/b] Along the railway there are kilometer posts at a distance of $1$ km from each other. One of them was painted yellow and six were painted red. The sum of the distances from the yellow pillar to all the red ones is $14$ km. What is the maximum distance between the red pillars? [b]p10.[/b] The island nation is located on $100$ islands connected by bridges, with some islands also connected to the mainland by a bridge. It is known that from each island you can travel to each (possibly through other islands). In order to improve traffic safety, one-way traffic was introduced on all bridges. It turned out that from each island you can leave only one bridge and that from at least one of the islands you can go to the mainland. Prove that from each island you can get to the mainland, and along a single route. PS. You should use hide for answers. Collected [url=https://artofproblemsolving.com/community/c2416727_soros_olympiad_in_mathematics]here.[/url]

Prove that $5^n-3^n$ is not divisible by $2^n+65$ for any positive integer $n$.

The integers $1, 2, 3, \ldots, 2016$ are written on a board. You can choose any two numbers on the board and replace them with their average. For example, you can replace $1$ and $2$ with $1.5$, or you can replace $1$ and $3$ with a second copy of $2$. After $2015$ replacements of this kind, the board will have only one number left on it. (a) Prove that there is a sequence of replacements that will make the final number equal to $2$. (b) Prove that there is a sequence of replacements that will make the final number equal to $1000$.

Show that the determinant: \[ \begin{vmatrix} 0 & a & b & c \\ -a & 0 & d & e \\ -b & -d & 0 & f \\ -c & -e & -f & 0 \end{vmatrix} \] is non-negative, if its elements $a, b, c,$ etc., are real.

There are $60$ friends who want to visit each others home during summer vacation. Everyday, they decide to either stay home or visit the home of everyone who stayed home that day. Find the minimum number of days required for everyone to have visited their friends’ homes.

Let $n \geq 3$ be an integer. Rowan and Colin play a game on an $n \times n$ grid of squares, where each square is colored either red or blue. Rowan is allowed to permute the rows of the grid and Colin is allowed to permute the columns. A grid coloring is [i]orderly[/i] if: [list] [*]no matter how Rowan permutes the rows of the coloring, Colin can then permute the columns to restore the original grid coloring; and [*]no matter how Colin permutes the columns of the coloring, Rowan can then permute the rows to restore the original grid coloring. [/list] In terms of $n$, how many orderly colorings are there? [i]Proposed by Alec Sun[/i]

Find the largest positive integer $n$ not divisible by $10$ which is a multiple of each of the numbers obtained by deleting two consecutive digits (neither of them in the first or last position) of $n$. (Note: $n$ is written in the usual base ten notation.)

Three flower beds overlap as shown. Bed A has 500 plants, bed B has 450 plants, and bed C has 350 plants. Beds A and B share 50 plants, while beds A and C share 100. The total number of plants is [asy] draw((0,0)--(3,0)--(3,1)--(0,1)--cycle); draw(circle((.3,-.1),.7)); draw(circle((2.8,-.2),.8)); label("A",(1.3,.5),N); label("B",(3.1,-.2),S); label("C",(.6,-.2),S);[/asy] $ \text{(A)}\ 850\qquad\text{(B)}\ 1000\qquad\text{(C)}\ 1150\qquad\text{(D)}\ 1300\qquad\text{(E)}\ 1450 $

Let $P(x) =(x+1)^p (x-3)^q=x^n+a_1x^{n-1}+a_2x^{n-2}+...+a_{n-1}x+a_n$ where $p$ and $q$ are positive integers a) Given $a_1=a_2$, prove that $3n$ is a perfect square. b) Prove that there exist infinitely many pairs $(p, q)$ of positive integers p and q such that the equality $a_1=a_2$ is valid for the polynomial $P(x)$. (D. Bazylev)

In a football tournament there are $8$ teams, each of which plays exacly one match against every other team. If a team $A$ defeats team $B$, then $A$ is awarded $3$ points and $B$ gets $0$ points. If they end up in a tie, they receive $1$ point each. It turned out that in this tournament, whenever a match ended up in a tie, the two teams involved did not finish with the same final score. Find the maximum number of ties that could have happened in such a tournament.

The princess wishes to have a bracelet with $r$ rubies and $s$ emeralds arranged in such order that there exist two jewels on the bracelet such that starting with these and enumerating the jewels in the same direction she would obtain identical sequences of jewels. Prove that it is possible to fulfill the princess’s wish if and only if $r$ and $s$ have a common divisor.

Determine the smallest real number $C$ such that the inequality $$\frac x{\sqrt{yz}}\cdot\frac1{x+1}+\frac y{\sqrt{zx}}\cdot\frac1{y+1}+\frac z{\sqrt{xy}}\cdot\frac1{x+1}\le C$$holds for all positive real numbers $x,y$ and $z$ with $\frac1{x+1}+\frac1{y+1}+\frac1{z+1}=1$.

Let there be $2n$ positive reals $a_1,a_2,...,a_{2n}$. Let $s = a_1 + a_3 +...+ a_{2n-1}$, $t = a_2 + a_4 + ... + a_{2n}$, and $x_k = a_k + a_{k+1} + ... + a_{k+n-1}$ (indices are taken modulo $2n$). Prove that $$\frac{s}{x_1}+\frac{t}{x_2}+\frac{s}{x_3}+\frac{t}{x_4}+...+\frac{s}{x_{2n-1}}+\frac{t}{x_{2n}}>\frac{2n^2}{n+1}$$

Let $ABCD$ be a convex quadrilateral with positive integer side lengths, $\angle{A} = \angle{B} = 120^{\circ}, |AD - BC| = 42,$ and $CD = 98$. Find the maximum possible value of $AB$.

The formula expressing the relationship between $ x$ and $ y$ in the table is: \[ \begin{tabular}{|c|c|c|c|c|c|} \hline x & 2 & 3 & 4 & 5 & 6 \\ \hline y & 0 & 2 & 6 & 12 & 20 \\ \hline \end{tabular} \] $ \textbf{(A)}\ y \equal{} 2x \minus{} 4 \qquad\textbf{(B)}\ y \equal{} x^2 \minus{} 3x \plus{} 2 \qquad\textbf{(C)}\ y \equal{} x^3 \minus{} 3x^2 \plus{} 2x$ $ \textbf{(D)}\ y \equal{} x^2 \minus{} 4x \qquad\textbf{(E)}\ y \equal{} x^2 \minus{} 4$

Let $ABC$ be a triangle such that $AB = 7$, $BC = 8$, and $CA = 9$. There exists a unique point $X$ such that $XB = XC$ and $XA$ is tangent to the circumcircle of $ABC$. If $XA = \tfrac ab$, where $a$ and $b$ are coprime positive integers, find $a + b$. [i]Proposed by Alexander Wang[/i]

Alan, Barb, Cory, and Doug are on the golf team, Doug, Emma, Fran, and Greg are on the swim team, and Greg, Hope, Inga, and Alan are on the tennis team. These nine people sit in a circle in random order. The probability that no two people from the same team sit next to each other is $\tfrac{m}{n}$ where $m$ and $n$ are relatively prime positive integers. Find $m + n.$

For each positive integer $\,n,\;S(n)\,$ is defined to be the greatest integer such that, for every positive integer $\,k\leq S(n),\;n^{2}\,$ can be written as the sum of $\,k\,$ positive squares. [b]a.)[/b] Prove that $\,S(n)\leq n^{2}-14\,$ for each $\,n\geq 4$. [b]b.)[/b] Find an integer $\,n\,$ such that $\,S(n)=n^{2}-14$. [b]c.)[/b] Prove that there are infintely many integers $\,n\,$ such that $S(n)=n^{2}-14.$

Does there exist a convex polyhedron in which each internal angle of each of its faces is either a right angle or an obtuse angle, and which has exactly $100$ edges? Justify your answer.

We say a finite set $S$ of points with $|S|\ge3$ is [i]good[/i] if for any three distinct elements of $S$, they are non-collinear and the orthocenter of them is also in $S$. Find all good sets.

a)Find a matrix $A\in \mathcal{M}_3(\mathbb{C})$ such that $A^2\neq O_3$ and $A^3=O_3$. b)Let $n,p\in\{2,3\}$. Prove that if there is bijective function $f:\mathcal{M}_n(\mathbb{C})\rightarrow \mathcal{M}_p(\mathbb{C})$ such that $f(XY)=f(X)f(Y),\ \forall X,Y\in \mathcal{M}_n(\mathbb{C})$, then $n=p$. [i]Ion Savu[/i]