Let $ABCD$ be a cyclic quadrilateral such that $AB = AD + BC$ and $CD < AB$. The diagonals $AC$ and $BD$ intersect at $P$, while the lines $AD$ and $BC$ intersect at $Q$. The angle bisector of $\angle APB$ meets $AB$ at $T$. Show that the circumcenter of the triangle $CTD$ lies on the circumcircle of the triangle $CQD$. [i]Proposed by Nikola Velov[/i]

Let be a triangle $ ABC, $ and three points $ A',B',C' $ on the segments $ BC,CA, $ respectively, $ AB, $ such that the lines $ AA',BB',CC' $ are concurent at $ M. $ Name $ a,b,c,x,y,z $ the areas of the triangles $ AB'M,BC'M,CA'M,AC'M,BA'M, $ respectively, $ CB'M. $ Show that: [b]a)[/b] $ abc=xyz $ [b]b)[/b] $ ab+bc+ca=xy+yz+zx $ [i]Bogdan Enescu[/i] and [i]Marcel Chiriță[/i]

A cross with length $p$ (or [i]p-cross[/i] for short) will be called the figure formed by a unit square and 4 rectangles $p-1$ x $1$ on its sides. What’s the least amount of colors one has to use to color the cells of an infinite table, so that each [i]p-cross[/i] on it covers cells, no two of which are in the same color?

Two years ago Tom was $25\%$ shorter than Mary. Since then Tom has grown $20\%$ taller, and Mary has grown $4$ inches taller. Now Mary is $20\%$ taller than Tom. How many inches tall is Tom now?

Given an arbitrary tetrahedron. Prove that its six edges can be divided into two triplets so that from each triple it was possible to form a triangle.

Let $p \ge 5$ be a prime number. Prove that there exists an integer $a$ with $1 \le a \le p-2$ such that neither $a^{p-1} -1$ nor $(a+1)^{p-1} -1$ is divisible by $p^2$.

Let $x,y$ be real numbers with $x \geqslant \sqrt{2021}$ such that \[ \sqrt[3]{x+\sqrt{2021}}+\sqrt[3]{x-\sqrt{2021}} = \sqrt[3]{y}\] Determine the set of all possible values of $y/x$.

Given positive numbers $a,b,c,d$. Prove that the set of inequalities $$a+b<c+d$$ $$(a+b)(c+d)<ab+cd$$ $$(a+b)cd<ab(c+d)$$ contain at least one wrong.

Let $(x_n)$ define by $x_1\in \left(0;\dfrac{1}{2}\right)$ and $x_{n+1}=3x_n^2-2nx_n^3$ for all $n\ge 1$. a) Prove that $(x_n)$ convergence to $0$. b) For each $n\ge 1$, let $y_n=x_1+2x_2+\cdots+n x_n$. Prove that $(y_n)$ has a limit.

Let $ s$ be the limiting sum of the geometric series $ 4\minus{} \frac83 \plus{} \frac{16}{9} \minus{} \dots$, as the number of terms increases without bound. Then $ s$ equals: $ \textbf{(A)}\ \text{a number between 0 and 1} \qquad \textbf{(B)}\ 2.4 \qquad \textbf{(C)}\ 2.5 \qquad \textbf{(D)}\ 3.6 \qquad \textbf{(E)}\ 12$

Let $d(n)$ be the number of positive divisors of a positive integer $n$. Let $\mathbb{N}$ be the set of all positive integers. Say that a function $F$ from $\mathbb{N}$ to $\mathbb{N}$ is [i]divisor-respecting[/i] if $d(F(mn)) = d(F(m)) d(F(n))$ for all positive integers $m$ and $n$, and $d(F(n)) \le d(n)$ for all positive integers $n$. Find all divisor-respecting functions. Justify your answer.

Consider the sequence $a_1, a_2, \dots$ of positive integers such that $a_1=2$ and $a_{n+1}=a_n^4+a_n^3-3a_n^2-a_n+2$, for all $n\geqslant 1$. Prove that there exist infinitely many prime numbers that don't divide any term of the sequence. [i]Proposed by Pavel Ciurea[/i]

Kayla draws three triangles on a sheet of paper. What is the maximum possible number of regions, including the exterior region, that the paper can be divided into by the sides of the triangles? [i]Proposed by Michael Tang[/i]

If $x,y,z\in\mathbb N$ and $xy=z^2+1$ prove that there exists integers $a,b,c,d$ such that $x=a^2+b^2$, $y=c^2+d^2$, $z=ac+bd$.

The graph shows velocity as a function of time for a car. What was the acceleration at time = $90$ seconds? [asy] size(275); pen dps = linewidth(0.7) + fontsize(10); defaultpen(dps); draw((0,0)--(6,0)); draw((0,1)--(6,1)); draw((0,2)--(6,2)); draw((0,3)--(6,3)); draw((0,4)--(6,4)); draw((0,0)--(0,4)); draw((1,0)--(1,4)); draw((2,0)--(2,4)); draw((3,0)--(3,4)); draw((4,0)--(4,4)); draw((5,0)--(5,4)); draw((6,0)--(6,4)); label("$0$",(0,0),S); label("$30$",(1,0),S); label("$60$",(2,0),S); label("$90$",(3,0),S); label("$120$",(4,0),S); label("$150$",(5,0),S); label("$180$",(6,0),S); label("$0$",(0,0),W); label("$10$",(0,1),W); label("$20$",(0,2),W); label("$30$",(0,3),W); label("$40$",(0,4),W); draw((0,0.6)--(0.1,0.55)--(0.8,0.55)--(1.2,0.65)--(1.9,1)--(2.2,1.2)--(3,2)--(4,3)--(4.45,3.4)--(4.5,3.5)--(4.75,3.7)--(5,3.7)--(5.5,3.45)--(6,3)); label("Time (s)", (7.5,0),S); label("Velocity (m/s)",(-1,3),W); [/asy] $ \textbf{(A)}\ 0.2\text{ m/s}^2\qquad\textbf{(B)}\ 0.33\text{ m/s}^2\qquad\textbf{(C)}\ 1.0\text{ m/s}^2\qquad\textbf{(D)}\ 9.8\text{ m/s}^2\qquad\textbf{(E)}\ 30\text{ m/s}^2 $

Let $ABCD$ be a cyclic quadrilaterla and let $E$ be the point of intersection of its diagonals $AC$ and $BD$. Suppose $AD$ and $BC$ meet in $F$. Let the midpoints of $AB$ and $CD$ be $G$ and $H$ respectively. If $\Gamma $ is the circumcircle of triangle $EGH$, prove that $FE$ is tangent to $\Gamma $.

David and Evan are playing a game. Evan thinks of a positive integer $N$ between 1 and 59, inclusive, and David tries to guess it. Each time David makes a guess, Evan will tell him whether the guess is greater than, equal to, or less than $N$. David wants to devise a strategy that will guarantee that he knows $N$ in five guesses. In David's strategy, each guess will be determined only by Evan's responses to any previous guesses (the first guess will always be the same), and David will only guess a number which satisfies each of Evan's responses. How many such strategies are there? Note: David need not guess $N$ within his five guesses; he just needs to know what $N$ is after five guesses.

Given is a positive integer $n$ divisible by $3$ and such that $2n-1$ is a prime. Does there exist a positive integer $x>n$ such that $$nx^{n+1}+(2n+1)x^n-3(n-1)x^{n-1}-x-3$$ is a product of the first few odd primes?

For each positive integer $n$, prove that there are two consecutive positive integers each of which is the product of $n$ positive integers greater than $1$.

Fill in each blank unshaded cell with a positive integer less than 100, such that every consecutive group of unshaded cells within a row or column is an arithmetic sequence. You do not need to prove that your answer is the only one possible; you merely need to find an answer that satisfies the constraints above. (Note: In any other USAMTS problem, you need to provide a full proof. Only in this problem is an answer without justification acceptable.) [asy] size(9cm); for (int x=0; x<=11; ++x) draw((x, 0) -- (x, 5), linewidth(.5pt)); for (int y=0; y<=5; ++y) draw((0, y) -- (11, y), linewidth(.5pt)); filldraw((0,4)--(0,3)--(2,3)--(2,4)--cycle, gray, gray); filldraw((1,1)--(1,2)--(3,2)--(3,1)--cycle, gray, gray); filldraw((4,1)--(4,4)--(5,4)--(5,1)--cycle, gray, gray); filldraw((7,0)--(7,3)--(6,3)--(6,0)--cycle, gray, gray); filldraw((7,4)--(7,5)--(6,5)--(6,4)--cycle, gray, gray); filldraw((8,1)--(8,2)--(10,2)--(10,1)--cycle, gray, gray); filldraw((9,4)--(9,3)--(11,3)--(11,4)--cycle, gray, gray); draw((0,0)--(11,0)--(11,5)--(0,5)--cycle); void foo(int x, int y, string n) { label(n, (x+0.5, y+0.5)); } foo(1, 2, "10"); foo(4, 0, "31"); foo(5, 0, "26"); foo(10, 0, "59"); foo(0, 4, "3"); foo(7, 4, "59"); [/asy]

And it came to pass that Jeb owned over a thousand chickens. So Jeb counted his chickens. And Jeb reported the count to Hannah. And Hannah reported the count to Joshua. And Joshua reported the count to Caleb. And Caleb reported the count to Rachel. But as fate would have it, Jeb had over-counted his chickens by nine chickens. Then Hannah interchanged the last two digits of the count before reporting it to Joshua. And Joshua interchanged the first and the third digits of the number reported to him before reporting it to Caleb. Then Caleb doubled the number reported to him before reporting it to Rachel. Now it so happens that the count reported to Rachel was the correct number of chickens that Jeb owned. How many chickens was that?

Find the largest number $\lambda$ such that $a^2+b^2+c^2+d^2 \geq ab + \lambda bc + cd$ for all real numbers $a,b,c,d$

Let $a,b \in R^+$ with $ab = 1$, prove that $$\frac{1}{a^3 + 3b}+\frac{1}{b^3 + 3a}\le \frac12.$$

A [i]lattice point[/i] in the Cartesian plane is a point whose coordinates are both integers. A [i]lattice polygon[/i] is a polygon all of whose vertices are lattice points. Let $\Gamma$ be a convex lattice polygon. Prove that $\Gamma$ is contained in a convex lattice polygon $\Omega$ such that the vertices of $\Gamma$ all lie on the boundary of $\Omega$, and exactly one vertex of $\Omega$ is not a vertex of $\Gamma$.

If $F(n)$ is a function such that $F(1)=F(2)=F(3)=1$, and such that $F(n+1)=\dfrac{F(n)\cdot F(n-1)+1}{F(n-2)}$ for $n\ge 3$, then $F(6)$ is equal to $\textbf{(A) }2\qquad\textbf{(B) }3\qquad\textbf{(C) }7\qquad\textbf{(D) }11\qquad \textbf{(E) }26$