Let $A(XYZ)$ be the area of the triangle $XYZ$. A non-regular convex polygon $P_1 P_2 \ldots P_n$ is called [i]guayaco[/i] if exists a point $O$ in its interior such that \[A(P_1OP_2) = A(P_2OP_3) = \cdots = A(P_nOP_1).\] Show that, for every integer $n \ge 3$, a guayaco polygon of $n$ sides exists.

Determine the number of ways to select a sequence of $ 8$ sets $ A_1,A_2,\ldots,A_8$, such that each is a subset (possibly empty) of $ \{1,2\}$ and $ A_m$ contains $ A_n$ if $ m$ divides $ n$.

Find all positive integers $n$ such that $n!+2$ divides $(2n)!$.

$ S\subset\mathbb N$ is called a square set, iff for each $ x,y\in S$, $ xy\plus{}1$ is square of an integer. a) Is $ S$ finite? b) Find maximum number of elements of $ S$.

How many positive solutions are there to $x^{10}+7x^9+14x^8+1729x^7-1379x^6=0$? How many positive integer solutions?

The numbers $16$ and $25$ are a pair of consecutive perfect squares whose difference is $9$. How many pairs of consecutive positive perfect squares have a difference of less than or equal to $2023$? $\textbf{(A) } 674 \qquad \textbf{(B) } 1011 \qquad \textbf{(C) } 1010 \qquad \textbf{(D) } 2019 \qquad \textbf{(E) } 2017$

Given a polynomial $P$ and a positive integer $k$, we denote the $k$-fold composition of $P$ by $P^{\circ k}$. A polynomial $P$ with real coefficients is called [b]perfect[/b] if for each integer $n$ there is a positive integer $k$ so that $P^{\circ k}(n)$ is an integer. Is it true that for each perfect polynomial $P$, there exists a positive $m$ so that for each integer $n$ there is $0<k\leq m$ for which $P^{\circ k}(n)$ is an integer?

We call [i]Pythagorean triple[/i] a triple $(x,y,z)$ of positive integers such that $x<y<z$ and $x^2+y^2=z^2$. Prove that for all $n \in \mathbb{N}$ number $2^{n+1}$ is in exactly $n$ [i]Pythagorean triples[/i]

David recently bought a large supply of letter tiles. One day he arrives back to his dorm to find that some of the tiles have been arranged to read $\textsc{Central Michigan University}$. What is the smallest number of tiles David must remove and/or replace so that he can rearrange them to read $\textsc{Carnegie Mellon University}$?

Let $X$ be a set with $|X| = n$ , and let $X_1 , X_2 ,... X_n$ be the $n$subsets eith $|X_j| \geq 2$, for $1 \leq j \leq n$. Suppose for each $2$ element subset $Y$ of $X$, there is a unique $j$ in the set $1,2,3....,n$ such that $Y \subset X_j$ . Prove that $X_j \cap X_k \not= \Phi$ for all $1 \leq j < k \leq n$

What is the average length in letters of a word in this question? $\text{(A) }3.5\qquad\text{(B) }4\qquad\text{(C) }4.5\qquad\text{(D) }5\qquad\text{(E) }5.5$

The real numbers $a,b,c$ with $bc\neq0$ satisfy $\frac{1-c^2}{bc}\geq0.$ Prove that $10(a^2+b^2+c^2-bc^3)\geq2ab+5ac.$

A [i] magic square[/i] is a $3\times 3$ grid of numbers in which the sums of the numbers in each row, column, and long diagonal are all equal. How many magic squares exist where each of the integers from $11$ to $19$ inclusive is used exactly once and two of the numbers are already placed as shown below? $\begin{tabular}{|l|l|l|l|} \hline & & 18 \\ \hline & 15 & \\ \hline & & \\ \hline \end{tabular}$

Sophie has $20$ indistinguishable pairs of socks in a laundry bag. She pulls them out one at a time. After pulling out $30$ socks, the expected number of unmatched socks among the socks that she has pulled out can be expressed in simplest form as $\tfrac{m}{n}$. Find $m+n$.

A man travels $ m$ feet due north at $ 2$ minutes per mile. He returns due south to his starting point at $ 2$ miles per minute. The average rate in miles per hour for the entire trip is: $ \textbf{(A)}\ 75\qquad \textbf{(B)}\ 48\qquad \textbf{(C)}\ 45\qquad \textbf{(D)}\ 24\qquad\\ \textbf{(E)}\ \text{impossible to determine without knowing the value of }{m}$

In my calculator, one of the keys from $1$ to $9$ does not work properly: when you press it, a digit between $1$ and $9$ appears on the screen that is not the correct one. When I tried to write the number $987654321$, a number divisible by $11$ appeared on the screen and leaves a remainder of $3$ when divided by $9$. What is the broken key? What is the number that appeared on the screen?

For an infinite sequence $a_1, a_2,. . .$ denote as it's [i]first derivative[/i] is the sequence $a'_n= a_{n + 1} - a_n$ (where $n = 1, 2,..$.), and her $k$- th derivative as the first derivative of its $(k-1)$-th derivative ($k = 2, 3,...$). We call a sequence [i]good[/i] if it and all its derivatives consist of positive numbers. Prove that if $a_1, a_2,. . .$ and $b_1, b_2,. . .$ are good sequences, then sequence $a_1\cdot b_1, a_2 \cdot b_2,..$ is also a good one. R. Salimov

What is the least possible value of $(xy-1)^2+(x+y)^2$ for real numbers $x$ and $y$? $\textbf{(A)}\ 0 \qquad\textbf{(B)}\ \frac14 \qquad\textbf{(C)}\ \frac12 \qquad\textbf{(D)}\ 1 \qquad\textbf{(E)}\ 2$

[b]3/1/32.[/b] The bisectors of the internal angles of parallelogram $ABCD$ determine a quadrilateral with the same area as $ABCD$. Given that $AB > BC$, compute, with proof, the ratio $\frac{AB}{BC}$.

Given an infinite sequence of numbers $a_1, a_2,...$, in which there are no two equal members. Segment $a_i, a_{i+1}, ..., a_{i+m-1}$ of this sequence is called a monotone segment of length $m$, if $a_i < a_{i+1} <...<a_{i+m-1}$ or $a_i > a_{i+1} >... > a_{i+m-1}$. It turned out that for each natural $k$ the term $a_k$ is contained in some monotonic segment of length $k + 1$. Prove that there exists a natural $N$ such that the sequence $a_N , a_{N+1} ,...$ monotonic.

Distinct points $A$ and $B$ are on a semicircle with diameter $MN$ and center $C$. The point $P$ is on $CN$ and $\angle CAP = \angle CBP = 10^{\circ}$. If $\stackrel{\frown}{MA} = 40^{\circ}$, then $\stackrel{\frown}{BN}$ equals [asy] size(200); defaultpen(linewidth(0.7)+fontsize(10)); pair C=origin, N=dir(0), B=dir(20), A=dir(135), M=dir(180), P=(3/7)*dir(C--N); draw(M--N^^C--A--P--B--C^^Arc(origin,1,0,180)); markscalefactor=0.03; draw(anglemark(C,A,P)); draw(anglemark(C,B,P)); pair point=C; label("$A$", A, dir(point--A)); label("$B$", B, dir(point--B)); label("$C$", C, S); label("$M$", M, dir(point--M)); label("$N$", N, dir(point--N)); label("$P$", P, S); label("$40^\circ$", C+(-0.15,0), NW); label("$10^\circ$", B+(0,0.05), W); label("$10^\circ$", A+(0.05,0.02), E);[/asy] $ \textbf{(A)}\ 10^{\circ}\qquad\textbf{(B)}\ 15^{\circ}\qquad\textbf{(C)}\ 20^{\circ}\qquad\textbf{(D)}\ 25^{\circ}\qquad\textbf{(E)}\ 30^{\circ}$

The points $M$ and $N$ are on the side $BC$ of $\Delta ABC$, so that $BM=CN$ and $M$ is between $B$ and $N$. Points $P\in AN$ and $Q\in AM$ are such that $\angle PMC=\angle MAB$ and $\angle QNB=\angle NAC$. Prove that $\angle QBC=\angle PCB$.

Consider an infinite sequence of reals $x_1, x_2, x_3, ...$ such that $x_1 = 1$, $x_2 =\frac{2\sqrt3}{3}$ and with the recursive relationship $$n^2 (x_n - x_{n-1} - x_{n-2}) - n(3x_n + 2x_{n-1} + x_{n-2}) + (x_nx_{n-1}x_{n-2} + 2x_n) = 0.$$ Find $x_{2019}$.

There are $26$ students in the class. They agreed that each of them would either be a liar (liars always lie) or a knight (knights always tell the truth). When they came to the class and sat down for desks, each of them said: “I am sitting next to a liar.” Then some students moved for other desks. After that, everyone says: “ I am sitting next to a knight .” Is this possible? Every time exactly two students sat at any desk.

Prove that if the numbers $p_1, p_2, q_1, q_2$ satisfy the condition $$(q_1 - q_2)^2 + (p_1 - p_2)(p_1q_2 -p_2q_1)<0$$ then the square polynomials $x^2 + p_1x + q_1$ and $x^2 + p_2x + q_2$ have real roots, and between the roots of each there is a root of another one.