A [i]slab[/i] is the set of points strictly between two parallel planes. Prove that a countable sequence of slabs, the sum of whose thicknesses converges, cannot fill space.

Natural numbers $ a \le b \le c \le d$ satisfy $ a\plus{}b\plus{}c\plus{}d\equal{}30$. Find the maximum value of the product $ P\equal{}abcd.$

The function $f$ is defined on the set $\mathbb{Q}$ of all rational numbers and has values in $\mathbb{Q}$. It satisfies the conditions $f(1) = 2$ and $f(xy) = f(x)f(y) - f(x+y) + 1$ for all $x,y \in \mathbb{Q}$. Determine $f$.

Let $ABC$ be a triangle with incenter $I$, and $A$-excenter $\Gamma$. Let $A_1,B_1,C_1$ be the points of tangency of $\Gamma$ with $BC,AC$ and $AB$, respectively. Suppose $IA_1, IB_1$ and $IC_1$ intersect $\Gamma$ for the second time at points $A_2,B_2,C_2$, respectively. $M$ is the midpoint of segment $AA_1$. If the intersection of $A_1B_1$ and $A_2B_2$ is $X$, and the intersection of $A_1C_1$ and $A_2C_2$ is $Y$, prove that $MX=MY$.

In the $8 \times 8$ grid below, label $8$ squares with $X$ and 8 squares with $Y$ such that: 1. No square can be labeled with both an $X$ and a $Y$. 2. Each row and each column must contain exactly one square labeled $X$ and one square labeled $Y$. 3. Any square marked with a $?$ or a $\heartsuit$ cannot be labeled with an $X$ or a $Y$. 4. We say that a square marked with a $?$ or a $\heartsuit$ sees a label ($X$ or $Y$) if one can move in a straight line horizontally or vertically from the marked square to the square with the label, without crossing any other squares with $X$’s or $Y$’s. It is OK to cross other squares marked with a $?$ or $\heartsuit$. Using this definition: (a) Each square marked with a $?$ must see exactly 2 $X$’s and 1 $Y$. (b) Each square marked with a $\heartsuit$ must see exactly 1 $X$ and 2 $Y$’s. \begin{tabular}{ | c | c | c | c | c | c | c | c | } \hline & & $\star$ & $\star$ & $\star$ & $\star$ & $\star$ & \\ \hline & & & & & & & $\star$ \\ \hline & & & & & & & $\star$ \\ \hline & $\heartsuit$ & & & & & & $\star$ \\ \hline & & & & & & & $\star$ \\ \hline & & & & & & & $\heartsuit$ \\ \hline & & & & $\star$ & & & \\ \hline & & & & & & & \\ \hline \end{tabular} There is a unique solution, but you do not need to prove that your answer is the only one possible. You merely need to find an answer that satisfies the conditions of the problem. (Note: In any other USAMTS problem, you need to provide a full proof. Only in this problem is an answer without justification acceptable.)

A soccer ball is usually made from a polyhedral fugure model, with two types of faces, hexagons and pentagons, and in every vertex incide three faces - two hexagons and one pentagon. We call a polyhedron [i]soccer-ball[/i] if it is similar to the traditional soccer ball, in the following sense: its faces are \(m\)-gons or \(n\)-gons, \(m \not= n\), and in every vertex incide three faces, two of them being \(m\)-gons and the other one being an \(n\)-gon. [list='i'] [*] Show that \(m\) needs to be even. [*] Find all soccer-ball polyhedra. [/list]

Find all pair $(x,y)$ of positive integers satisfying $$\left|\frac{x}{y}-\sqrt2\right|<\frac{1}{y^3}.$$

Prove that if $a, b, c$ are positive numbers and $ab + bc + ca > a+ b + c$, then $a + b + c > 3$.

A quadrilateral and a pentagon (both not self-intersecting) intersect each other at $N$ distinct points, where $N$ is a positive integer. What is the maximal possible value of $N$? [i]Proposed by James Lin

(a) If $m, n$ are positive integers such that $2^n-1$ divides $m^2 + 9$, prove that $n$ is a power of $2$; (b) If $n$ is a power of $2$, prove that there exists a positive integer $m$ such that $2^n-1$ divides $m^2 + 9$.

A point $D$ lies on the side $BC$ , and a point $E$ on the side $AC$ , of the triangle $ABC$ , and $BD$ and $AE$ have the same length. The line through the centres of the circumscribed circles of the triangles $ADC$ and $BEC$ crosses $AC$ in $K$ and $BC$ in $L$. Show that $KC$ and $LC$ have the same length.

Given points $O$ and $A$ in the plane. Every point in the plane is colored with one of a finite number of colors. Given a point $X$ in the plane, the circle $C(X)$ has center $O$ and radius $OX+{\angle AOX\over OX}$, where $\angle AOX$ is measured in radians in the range $[0,2\pi)$. Prove that we can find a point $X$, not on $OA$, such that its color appears on the circumference of the circle $C(X)$.

Which of the following inequalities are satisfied for all real numbers $ a$, $ b$, $ c$, $ x$, $ y$, $ z$ which satisfy the conditions $ x < a$, $ y < b$, and $ z < c$? $ \text{I}. \ xy \plus{} yz \plus{} zx < ab \plus{} bc \plus{} ca$ $ \text{II}. \ x^2 \plus{} y^2 \plus{} z^2 < a^2 \plus{} b^2 \plus{} c^2$ $ \text{III}. \ xyz < abc$ $ \textbf{(A)}\ \text{None are satisfied.} \qquad \textbf{(B)}\ \text{I only} \qquad \textbf{(C)}\ \text{II only} \qquad$ $ \textbf{(D)}\ \text{III only} \qquad \textbf{(E)}\ \text{All are satisfied.}$

Let $a_1, a_2, \cdots , a_n$ and $b_1, b_2,\cdots, b_n$ be nonnegative real numbers. Show that \[(a_1a_2 \cdots a_n)^{1/n}+ (b_1b_2 \cdots b_n)^{1/n} \le ((a_1 + b_1)(a_2 + b_2) \cdots (a_n + b_n))^{1/n}\]

Points $C_0$ and $B_0$ are the respective midpoints of sides $AB$ and $AC$ of a non-isosceles acute triangle $ABC$, $O$ is its circumscenter and $H$ is the orthocenter. Lines $BH$ and $OC_0$ intersect at $P$, while lines $CH$ and $OB_0$ intersect at $Q$. $OPHQ$ is rhombus. Prove that points $A$, $P$ and $Q$ are collinear. (Author: L. Emelyanov)

The chord $[CD]$ of the circle with diameter $[AB]$ is perpendicular to $[AB]$. Let $M$ and $N$ be the midpoints of $[BC]$ and $[AD]$, respectively. If $|BC|=6$ and $|AD|=2\sqrt{3}$, then $|MN|=?$ $ \textbf{(A)}\ 4 \qquad \textbf{(B)}\ 3 \sqrt 2 \qquad \textbf{(C)}\ \sqrt{21} \qquad \textbf{(D)}\ 5 \qquad \textbf{(E)}\ \text{None}$

Let $ S\subseteq\mathbb{R}$ be a set of real numbers. We say that a pair $ (f, g)$ of functions from $ S$ into $ S$ is a [i]Spanish Couple[/i] on $ S$, if they satisfy the following conditions: (i) Both functions are strictly increasing, i.e. $ f(x) < f(y)$ and $ g(x) < g(y)$ for all $ x$, $ y\in S$ with $ x < y$; (ii) The inequality $ f\left(g\left(g\left(x\right)\right)\right) < g\left(f\left(x\right)\right)$ holds for all $ x\in S$. Decide whether there exists a Spanish Couple [list][*] on the set $ S \equal{} \mathbb{N}$ of positive integers; [*] on the set $ S \equal{} \{a \minus{} \frac {1}{b}: a, b\in\mathbb{N}\}$[/list] [i]Proposed by Hans Zantema, Netherlands[/i]

Let $ABC$ be a right-angled triangle with $\angle B = 90^{\circ}$. Let the length of the altitude $BD$ be equal to $12$. What is the minimum possible length of $AC$, given that $AC$ and the perimeter of triangle $ABC$ are integers?

Find all continuous functions $f : R \to R$ such that $$f(x + f(y)) = f(x + y) + y,$$ for all $x, y \in R$. No proof is required for this problem.

Find the last three digits in the product $1 \cdot 3\cdot 5\cdot 7 \cdot . . . \cdot 2009 \cdot 2011$.

Find all pairs of positive integers $(x, y)$ so that $$(xy - 6)^2 | x^2 + y^2$$

Given any set $S$ of positive integers, show that at least one of the following two assertions holds: (1) There exist distinct finite subsets $F$ and $G$ of $S$ such that $\sum_{x\in F}1/x=\sum_{x\in G}1/x$; (2) There exists a positive rational number $r<1$ such that $\sum_{x\in F}1/x\neq r$ for all finite subsets $F$ of $S$.

A small bottle of shampoo can hold 35 milliliters of shampoo, whereas a large bottle can hold 500 milliliters of shampoo. Jasmine wants to buy the minimum number of small bottles necessary to completely fill a large bottle. How many bottles must she buy? $ \textbf{(A)}\ 11 \qquad \textbf{(B)}\ 12 \qquad \textbf{(C)}\ 13 \qquad \textbf{(D)}\ 14 \qquad \textbf{(E)}\ 15$

Compute the smallest nonnegative integer that can be written as the sum of 2020 distinct integers.

The Lucas numbers $L_n$ are defined recursively as follows: $L_0=2,L_1=1,L_n=L_{n-1}+L_{n-2}$ for $n\geq2$. Let $r=0.21347\dots$, whose digits form the pattern of the Lucas numbers. When the numbers have multiple digits, they will "overlap," so $r=0.2134830\dots$, [b]not[/b] $0.213471118\dots$. Express $r$ as a rational number $\frac{p}{q}$, where $p$ and $q$ are relatively prime.