Found problems: 85335
An arbitary triangle is partitioned to some triangles homothetic with itself. The ratio of homothety of the triangles can be positive or negative.
Prove that sum of all homothety ratios equals to $1$.
Time allowed for this problem was 45 minutes.
Fill in the grid with the numbers 1 to 6 so that each number appears exactly once in
each row and column. A horizontal gray line marks any cell when it is the middle cell of
the three consecutive cells with the largest sum in that row. Similarly, a vertical gray line
marks any cell when it is the middle of the three consecutive cells with the largest sum in
that column. If there is a tie, multiple lines are drawn in the row or column. A cell can have
both lines drawn, with the appearance of a plus sign.
[asy]
// Change this to see the solution
bool DRAW_SOLUTION = true;
int n = 6;
real LINE_WIDTH = 0.3;
void drawHLine(int x, int y) {
fill((x,y+0.5-LINE_WIDTH/2)--(x,y+0.5+LINE_WIDTH/2)--(x+1,y+0.5+LINE_WIDTH/2)--(x+1,y+0.5-LINE_WIDTH/2)--cycle, gray(0.8));
}
void drawVLine(int x, int y) {
fill((x+0.5-LINE_WIDTH/2,y)--(x+0.5+LINE_WIDTH/2,y)--(x+0.5+LINE_WIDTH/2,y+1)--(x+0.5-LINE_WIDTH/2,y+1)--cycle, gray(0.8));
}
void drawNum(int x, int y, int num) {
label(scale(1.5)*string(num), (x+0.5,y+0.5));
}
void drawSolNum(int x, int y, int num) {
if (DRAW_SOLUTION) {
drawNum(x, y, num);
}
}
drawHLine(2,0);
drawHLine(4,1);
drawHLine(1,2);
drawHLine(3,2);
drawHLine(4,3);
drawHLine(2,4);
drawHLine(3,5);
drawVLine(0,4);
drawVLine(1,3);
drawVLine(2,1);
drawVLine(2,3);
drawVLine(3,4);
drawVLine(4,1);
drawVLine(5,2);
drawNum(0, 0, 5);
drawNum(4, 0, 3);
drawNum(1, 2, 2);
drawNum(3, 3, 4);
for(int i = 0; i <= 6; i += 1) {
draw((i,0)--(i,6));
draw((0,i)--(6,i));
}
[/asy]
$9$ points are given in the interior of the unit square.
Prove there exists a triangle of area $\le \frac18$ whose vertices are three of the points.
Let $a,b,n$ be positive integers such that $2^n - 1 =ab$. Let $k \in \mathbb N$ such that $ab+a-b-1 \equiv 0 \pmod {2^k}$ and $ab+a-b-1 \neq 0 \pmod {2^{k+1}}$. Prove that $k$ is even.
In each square of a $2012\times 2012$ board there's a person. People are either honest, who always tell the truth, or liars, who always lie. At a given moment, each person makes the same statement: "In my row there are the same number of liars as in my column." Determine the minimum number of honest people that can be on the board.
Let $A_1A_2A_3A_4$ be a tetrahedron. Consider the sphere centered at $A_1$ which is tangent to the face $A_2A_3A_4$ of the tetrahedron. Show that the surface area of the part of the sphere which is inside the tetrahedron is less than the area of the triangle $A_2A_3A_4$.
[i]Sorin Rădulescu[/i]
A sequence $a_n$ is defined by $a_0 = 0$, and for all $n \ge 1$, $a_n = a_{n-1} + (-1)^n \cdot n^2$. Compute $a_{100}$
4. Given is an acute-angled triangle $A B C, H$ is its orthocenter. Let $P$ be an arbitrary point inside (and not on the sides) of the triangle $A B C$ that belongs to the circumcircle of the triangle $A B H$. Let $A^{\prime}, B^{\prime}$, $C^{\prime}$ be projections of point $P$ to the lines $B C, C A, A B$. Prove that the circumcircle of the triangle $A^{\prime} B^{\prime} C^{\prime}$ passes through the midpoint of segment $C P$.
Alexey Zaslavsky
Positive integers $x$ and $y$ satisfy the equation $\sqrt{x}+\sqrt{y}=\sqrt{1183}.$ What is the minimum possible value of $x+y?$
$\textbf{(A) }585 \qquad\textbf{(B) }595\qquad\textbf{(C) }623\qquad\textbf{(D) }700\qquad\textbf{(E) }791$
Let $\varphi(k)$ denote the numbers of positive integers less than or equal to $k$ and relatively prime to $k$. Prove that for some positive integer $n$, \[ \varphi(2n-1) + \varphi(2n+1) < \frac{1}{1000} \varphi(2n). \][i]Proposed by Evan Chen[/i]
A circle rolls along a side of an equilateral triangle. The radius of the circle is equal to the height of the triangle. Prove that the measure of the arc intercepted by the sides of the triangle on this circle is equal to $60^o$ at all times.
Given natural numbers $a,b$ and function $f: \mathbb{N} \to \mathbb{N} $ such that for any natural number $n, f\left( n+a \right)$ is divided by $f\left( {\left[ {\sqrt n } \right] + b} \right)$. Prove that for any natural $n$ exist $n$ pairwise distinct and pairwise relatively prime natural numbers ${{a}_{1}}$, ${{a}_{2}}$, $\ldots$, ${{a}_{n}}$ such that the number $f\left( {{a}_{i+1}} \right)$ is divided by $f\left( {{a}_{i}} \right)$ for each $i=1,2, \dots ,n-1$ .
(Here $[x]$ is the integer part of number $x$, that is, the largest integer not exceeding $x$.)
Let $ n$ be a positive integer and let $ a_1,a_2,a_3,\ldots,a_k$ $ ( k\ge 2)$ be distinct integers in the set $ { 1,2,\ldots,n}$ such that $ n$ divides $ a_i(a_{i + 1} - 1)$ for $ i = 1,2,\ldots,k - 1$. Prove that $ n$ does not divide $ a_k(a_1 - 1).$
[i]Proposed by Ross Atkins, Australia [/i]
Determine all polynomials $p$ with real coefficients for which $p(0)=0$ and
$$f(f(n))+n=4f(n)\qquad\text{for all }n\in\mathbb N,$$where $f(n)=\lfloor p(n)\rfloor$.
A $24$-hour digital clock shows times $h : m : s$, where $h$, $m$, and $s$ are integers with $0 \le h \le 23$, $0 \le m \le 59$, and $0 \le s \le 59$. How many times $h : m : s$ satisfy $h + m = s$?
A uniform pool ball of radius $r$ and mass $m$ begins at rest on a pool table. The ball is given a horizontal impulse $J$ of fixed magnitude at a distance $\beta r$ above its center, where $-1 \le \beta \le 1$. The coefficient of kinetic friction between the ball and the pool table is $\mu$. You may assume the ball and the table are perfectly rigid. Ignore effects due to deformation. (The moment of inertia about the center of mass of a solid sphere of mass $m$ and radius $r$ is $I_{cm} = \frac{2}{5}mr^2$.)
[asy]
size(250);
pen dps = linewidth(0.7) + fontsize(10); defaultpen(dps);
filldraw(circle((0,0),1),gray(.8));
draw((-3,-1)--(3,-1));
draw((-2.4,0.1)--(-2.4,0.6),EndArrow);
draw((-2.5,0)--(2.5,0),dashed);
draw((-2.75,0.7)--(-0.8,0.7),EndArrow);
label("$J$",(-2.8,0.7),W);
label("$\beta r$",(-2.3,0.35),E);
draw((0,-1.5)--(0,1.5),dashed);
draw((1.7,-0.1)--(1.7,-0.9),BeginArrow,EndArrow);
label("$r$",(1.75,-0.5),E);
[/asy]
(a) Find an expression for the final speed of the ball as a function of $J$, $m$, and $\beta$.
(b) For what value of $\beta$ does the ball immediately begin to roll without slipping, regardless of the value of $\mu$?
There is a right triangle $\triangle ABC$ in which $\angle A$ is the right angle. On side $AB$, there are three points $X$, $Y$, and $Z$ that satisfy $\angle ACX=\angle XCY=\angle YCZ=\angle ZCB$ and $BZ=2AX$. The smallest angle of $\triangle ABC$ is $\tfrac ab$ degrees, where $a,b$ are positive integers such that $\gcd(a,b)=1$. Find $a+b$.
A chord is drawn on a circle by choosing two points uniformly at random along its circumference. This is done two more times to obtain three total random chords. The circle is cut along these three lines, splitting it into pieces. The probability that one of the pieces is a triangle is $\frac{m}{n}$ , where $m$, $n$ are positive integers and gcd $(m,n) = 1$. Find $100m + n$.
The diagonals of a quadrilateral $ ABCD$ are perpendicular: $ AC \perp BD.$ Four squares, $ ABEF,BCGH,CDIJ,DAKL,$ are erected externally on its sides. The intersection points of the pairs of straight lines $ CL, DF, AH, BJ$ are denoted by $ P_1,Q_1,R_1, S_1,$ respectively (left figure), and the intersection points of the pairs of straight lines $ AI, BK, CE DG$ are denoted by $ P_2,Q_2,R_2, S_2,$ respectively (right figure). Prove that $ P_1Q_1R_1S_1 \cong P_2Q_2R_2S_2$ where $ P_1,Q_1,R_1, S_1$ and $ P_2,Q_2,R_2, S_2$ are the two quadrilaterals.
[i]Alternative formulation:[/i] Outside a convex quadrilateral $ ABCD$ with perpendicular diagonals, four squares $ AEFB, BGHC, CIJD, DKLA,$ are constructed (vertices are given in counterclockwise order). Prove that the quadrilaterals $ Q_1$ and $ Q_2$ formed by the lines $ AG, BI, CK, DE$ and $ AJ, BL, CF, DH,$ respectively, are congruent.
A $\textit{lattice point}$ of the coordinate plane is a point $(x,y)$ in which both $x$ and $y$ are integers. Let $k\geq2$ be a positive integer. Find the smallest positive integer $c_k$ (which may depend on $k$) such that every lattice point can be colored with one of $c_k$ colors, subject to the following two conditions:
[list=1]
[*] If $(x,y)$ and $(a,b)$ are two distinct neighboring points; that is, $|x-a|\leq1$ and $|y-b|\leq1$, then $(x,y)$ and $(a,b)$ must be different colors. [/*]
[*] If $(x,y)$ and $(a,b)$ are two lattice points such that $x\equiv a\pmod{k}$ and $y\equiv b\pmod{k}$, then $(x,y)$ and $(a,b)$ must be the same color. [/*]
[/list]
A subset $M$ of $\{1, 2, . . . , 2006\}$ has the property that for any three elements $x, y, z$ of $M$ with $x < y < z$, $x+ y$ does not divide $z$. Determine the largest possible size of $M$.
Let be a natural number $ k, $ and a matrix $ M\in\mathcal{M}_k(\mathbb{R}) $ having the property that
$$ \det\left( I-\frac{1}{n^2}\cdot A^2 \right) +1\ge\det \left( I -\frac{1}{n}\cdot A \right) +\det \left( I +\frac{1}{n}\cdot A \right) , $$
for all natural numbers $ n. $ Prove that the trace of $ A $ is $ 0. $
[i]Nelu Chichirim[/i]
Two triangles intersect to form seven finite disjoint regions, six of which are triangles with area 1. The last region is a hexagon with area \(A\). Compute the minimum possible value of \(A\).
[i]Proposed by Karthik Vedula[/i]
An odd positive integer $n$ can be expressed as the sum of two or more consecutive integers in exactly $2023$ ways. Find the greatest possible nonnegative integer $k$ such that $3^k$ is a factor of the least possible value of $n$.
Let $$z=\frac{1+i}{\sqrt{2}}.$$ What is $$(z^{1^2}+z^{2^2}+z^{3^2}+\dots+z^{{12}^2}) \cdot (\frac{1}{z^{1^2}}+\frac{1}{z^{2^2}}+\frac{1}{z^{3^2}}+\dots+\frac{1}{z^{{12}^2}})?$$
$\textbf{(A) } 18 \qquad \textbf{(B) } 72-36\sqrt2 \qquad \textbf{(C) } 36 \qquad \textbf{(D) } 72 \qquad \textbf{(E) } 72+36\sqrt2$