Can the set $\{1,2,...,33\}$ be partitioned into $11$ three-element sets, in each of which one element equals the sum of the other two?

Let $m,n\geq 2$ be integers. Let $f(x_1,\dots, x_n)$ be a polynomial with real coefficients such that $$f(x_1,\dots, x_n)=\left\lfloor \frac{x_1+\dots + x_n}{m} \right\rfloor\text{ for every } x_1,\dots, x_n\in \{0,1,\dots, m-1\}.$$ Prove that the total degree of $f$ is at least $n$.

Let $a_1, a_2,..., a_n$ be positive integers and $a$ positive integer greater than $1$ which is a multiple of the product $a_1a_2...a_n$. Prove that $a^{n+1} + a - 1$ is not divisible by $(a + a_1 -1)(a + a_2 - 1) ... (a + a_n -1)$.

Let $A_n$ be the number of ways to tile a $4 \times n$ rectangle using $2 \times 1$ tiles. Prove that $A_n$ is divisible by 2 if and only if $A_n$ is divisible by 3.

Let $a_1,a_2,\cdots,a_n,b_1,b_2,\cdots,b_n$ are real numbers in $[1,2]$. If $\sum_{i=1}^{n}a_i^2=\sum_{i=1}^{n}b_i^2$, prove that $$\sum_{i=1}^{n}\frac{a_i^3}{b_i}\leq\frac{17}{10}\sum_{i=1}^{n}a_i^2.$$ Find when the equality holds.

A simple pendulum of length $L$ is constructed from a point object of mass $m$ suspended by a massless string attached to a fixed pivot point. A small peg is placed a distance $2L/3$ directly below the fixed pivot point so that the pendulum would swing as shown in the figure below. The mass is displaced $5$ degrees from the vertical and released. How long does it take to return to its starting position? [asy] // Code by riben size(275); draw(circle((0,0),1),linewidth(2)); filldraw(circle((0,0),1),gray); draw((0,0)--(0,-70.8)); draw(circle((0,-71.8),3)); filldraw(circle((0,-71.8),3),gray); draw(circle((0,-45),1)); filldraw(circle((0,-45),1),gray); filldraw(circle((15,-70),3),gray,linewidth(0.2)); filldraw(circle((-15,-67),3),gray,linewidth(0.2)); draw((0,0)--(14.5,-66.5),dashed); draw((0,-45)--(-13,-65),dashed); // Labels label("Fixed Pivot Point",(0,0),4*E); label("Small Peg",(0,-45),12*E); label("Point Object of mass m",(0,-70),17*E); draw((-40,1)--(-40,-76.8),EndArrow(size=5)); draw((-40,-76.8)--(-40,1),EndArrow(size=5)); label("L",(-40,-37.9),E*2); [/asy] (A) $\pi \sqrt{\frac{L}{g}} \left(1+\sqrt{\frac{2}{3}}\right)$ (B) $\pi \sqrt{\frac{L}{g}} \left(2+\frac{2}{\sqrt{3}}\right)$ (C) $\pi \sqrt{\frac{L}{g}} \left(1+\frac{1}{3}\right)$ (D) $\pi \sqrt{\frac{L}{g}} \left(1+\sqrt{3}\right)$ (E) $\pi \sqrt{\frac{L}{g}} \left(1+\frac{1}{\sqrt{3}}\right)$

Suppose that $A = \left(\frac12, \sqrt3 \right)$. Suppose that $B, C, D$ are chosen on the ellipse $x^2 + (y/2)^2 = 1$ such that the area of $ABCD$ is maximized. Assume that $A, B, C, D$ lie on the ellipse going counterclockwise. What are all possible values of $B$?

A set $S=\{ s_1,s_2,...,s_k\}$ of positive real numbers is "polygonal" if $k\geq 3$ and there is a non-degenerate planar $k-$gon whose side lengths are exactly $s_1,s_2,...,s_k$; the set $S$ is multipolygonal if in every partition of $S$ into two subsets,each of which has at least three elements, exactly one of these two subsets in polygonal. Fix an integer $n\geq 7$. (a) Does there exist an $n-$element multipolygonal set, removal of whose maximal element leaves a multipolygonal set? (b) Is it possible that every $(n-1)-$element subset of an $n-$element set of positive real numbers be multipolygonal?

Given the true statement: If a quadrilateral is a square, then it is a rectangle. It follows that, of the converse and the inverse of this true statement is: $ \textbf{(A)}\ \text{only the converse is true} \qquad\textbf{(B)}\ \text{only the inverse is true }\qquad \textbf{(C)}\ \text{both are true} \qquad$ $\textbf{(D)}\ \text{neither is true} \qquad\textbf{(E)}\ \text{the inverse is true, but the converse is sometimes true} $

Let $m$ be a fixed integer greater than $1$. The sequence $x_0$, $x_1$, $x_2$, $\ldots$ is defined as follows: \[x_i = \begin{cases}2^i&\text{if }0\leq i \leq m - 1;\\\sum_{j=1}^mx_{i-j}&\text{if }i\geq m.\end{cases}\] Find the greatest $k$ for which the sequence contains $k$ consecutive terms divisible by $m$ . [i]Proposed by Marcin Kuczma, Poland[/i]

We define an operation $\oplus$ on the set $\{0, 1\}$ by \[ 0 \oplus 0 = 0 \,, 0 \oplus 1 = 1 \,, 1 \oplus 0 = 1 \,, 1 \oplus 1 = 0 \,.\] For two natural numbers $a$ and $b$, which are written in base $2$ as $a = (a_1a_2 \ldots a_k)_2$ and $b = (b_1b_2 \ldots b_k)_2$ (possibly with leading 0's), we define $a \oplus b = c$ where $c$ written in base $2$ is $(c_1c_2 \ldots c_k)_2$ with $c_i = a_i \oplus b_i$, for $1 \le i \le k$. For example, we have $7 \oplus 3 = 4$ since $ 7 = (111)_2$ and $3 = (011)_2$. For a natural number $n$, let $f(n) = n \oplus \left[ n/2 \right]$, where $\left[ x \right]$ denotes the largest integer less than or equal to $x$. Prove that $f$ is a bijection on the set of natural numbers.

Let $p = 10009$ be a prime number. Determine the number of ordered pairs of integers $(x,y)$ such that $1\le x,y \le p$ and $x^3-3xy+y^3+1$ is divisible by $p$.

Find all polynomials of degree 3, such that for each $x,y\geq 0$: \[p(x+y)\geq p(x)+p(y)\]

$ABC$ is an isosceles triangle such that $\angle{ABC}=90^\circ$ and $AB=2$. $D$ is the midpoint of $BC$ and $E$ is on $AC$ such that the area of $AEDB$ is twice the area of $ECD$. Find the length of $DE$.

Solve the equation $x^2 +7=2^n$ in integers.

Joe the Jaguar is on an infinite grid of unit squares, starting at the centre of one of them. At the $k$-th minute, Joe must jump a distance of $k$ units in any direction. For which $n$ is it possible for Joe to be inside or on the edge of the starting square after $n$ minutes?

Let $a_1,a_2,a_3,...$ be an infinite sequence of positive integers. Suppose that a sequence $a_1,a_2,\ldots$ of positive integers satisfies $a_1=1$ and \[a_{n}=\sum_{n\neq d|n}a_d\] for every integer $n>1$. Prove that the exist infinitely many integers $k$ such that $a_k=k$.

[b]p1.[/b] Find the smallest whole number $n \ge 2$ such that the product $(2^2 - 1)(3^2 - 1) ... (n^2 - 1)$ is the square of a whole number. [b]p2.[/b] The figure below shows a $ 10 \times 10$ square with small $2 \times 2$ squares removed from the corners. What is the area of the shaded region? [img]https://cdn.artofproblemsolving.com/attachments/7/5/a829487cc5d937060e8965f6da3f4744ba5588.png[/img] [b]p3.[/b] Three cars are racing: a Ford $[F]$, a Toyota $[T]$, and a Honda $[H]$. They began the race with $F$ first, then $T$, and $H$ last. During the race, $F$ was passed a total of $3$ times, $T$ was passed $5$ times, and $H$ was passed $8$ times. In what order did the cars finish? [b]p4.[/b] There are $11$ big boxes. Each one is either empty or contains $8$ medium-sized boxes inside. Each medium box is either empty or contains $8$ small boxes inside. All small boxes are empty. Among all the boxes, there are a total of $102$ empty boxes. How many boxes are there altogether? [b]p5.[/b] Ann, Mary, Pete, and finally Vlad eat ice cream from a tub, in order, one after another. Each eats at a constant rate, each at his or her own rate. Each eats for exactly the period of time that it would take the three remaining people, eating together, to consume half of the tub. After Vlad eats his portion there is no more ice cream in the tube. How many times faster would it take them to consume the tub if they all ate together? PS. You should use hide for answers. Collected [url=https://artofproblemsolving.com/community/c5h2760506p24143309]here[/url].

A barn with a flat roof is rectangular in shape, $ 10$ yd. wide, $ 13$ yd. long and 5 yd. high. It is to be painted inside and outside, and on the ceiling, but not on the roof or floor. The total number of sq. yd. to be painted is: $ \textbf{(A)}\ 360 \qquad\textbf{(B)}\ 460 \qquad\textbf{(C)}\ 490 \qquad\textbf{(D)}\ 590 \qquad\textbf{(E)}\ 720$

A Physicist for Fun discovered three types of very peculiar particles, and classified them as $P$, $H$ and $I$ particles. After months of study, this physicist discovered that he can join such particles and obtain new particles, according to the following operations: • A $P$ particle with an $H$ particle turns into one $I$ particle; • A $P$ particle with an $I$ particle turns into two $P$ particles and one $H$ particle; • An $H$ particle with an $I$ particle turns into four $P$ particles; Nothing happens when we try to join particles of the same type. It is also known that the physicist has $22$ $P$ particles, $21$ $H$ particles and $20$ $I$ particles. (a) After a finite number of operations, what is the largest possible number of particles that can be obtained? And what is the smallest possible number of particles? (b) Is it possible, after a finite number of operations, to obtain $22$ $P$ particles, $20$ $H$ particles, and $21$ $I$ particles? (c) Is it possible, after a finite number of operations, to obtain $34$ $H$ particles and $21$ $I$ particles?

Prove that if $a$ is an integer relatively prime with $35$ then $(a^4 - 1)(a^4 + 15a^2 + 1) \equiv 0$ mod $35$.

In a certain circle, the chord of a $d$-degree arc is 22 centimeters long, and the chord of a $2d$-degree arc is 20 centimeters longer than the chord of a $3d$-degree arc, where $d<120.$ The length of the chord of a $3d$-degree arc is $-m+\sqrt{n}$ centimeters, where $m$ and $n$ are positive integers. Find $m+n.$

Triangle $AB_0C_0$ has side lengths $AB_0 = 12$, $B_0C_0 = 17$, and $C_0A = 25$. For each positive integer $n$, points $B_n$ and $C_n$ are located on $\overline{AB_{n-1}}$ and $\overline{AC_{n-1}}$, respectively, creating three similar triangles $\triangle AB_nC_n \sim \triangle B_{n-1}C_nC_{n-1} \sim \triangle AB_{n-1}C_{n-1}$. The area of the union of all triangles $B_{n-1}C_nB_n$ for $n\geq1$ can be expressed as $\tfrac pq$, where $p$ and $q$ are relatively prime positive integers. Find $q$.

Several people came to the congress, each of whom has a certain number of tattoos on both hands. There are $n{}$ types of tattoos, and each of the $n{}$ types is found on the hands of at least $k{}$ people. For which pairs $(n, k)$ is it always possible for each participant to raise one of their hands so that all $n{}$ types of tattoos are present on the raised hands?

Determine all triples $(x, y,z)$ consisting of three distinct real numbers, that satisfy the following system of equations: $\begin {cases}x^2 + y^2 = -x + 3y + z \\ y^2 + z^2 = x + 3y - z \\ x^2 + z^2 = 2x + 2y - z \end {cases}$