Found problems: 85335
Prove that the sum of an odd number of vectors of length 1, of common origin $O$ and all situated in the same semi-plane determined by a straight line which goes through $O,$ is at least 1.
The numbers $p,q>0$ are given. Construct a rectangle $ABCD$ with $AE=p,AF=q$ where $E,F$ are midpoints of $BC,CD,$ respectively. Discuss conditions of solvability.
There are $2017$ lines in the plane such that no three of them go through the same point. Turbo the snail sits on a point on exactly one of the lines and starts sliding along the lines in the following fashion: she moves on a given line until she reaches an intersection of two lines. At the intersection, she follows her journey on the other line turning left or right, alternating her choice at each intersection point she reaches. She can only change direction at an intersection point. Can there exist a line segment through which she passes in both directions during her journey?
The board features a triangle $ABC$, its center of the circle circumscribed is the point $O$, the midpoint of the side $BC$ is the point $F$, and also some point $K$ on side $AC$ (see fig.). Master knowing that $\angle BAC$ of this triangle is equal to the sharp angle $\alpha$ has separately drawn an angle equal to $\alpha$. After this teacher wiped the board, leaving only the points $O, F, K$ and the angle $\alpha$. Is it possible with a compass and a ruler to construct the triangle $ABC$ ? Justify the answer.
(Grigory Filippovsky)
[img]https://1.bp.blogspot.com/-RRPt8HbqW4I/XObthZFXyyI/AAAAAAAAKOo/zfHemPjUsI4XAfV_tcmKA6_al0i_gQ9iACK4BGAYYCw/s1600/Yasinsky%2B2019%2Bp6.png[/img]
Find all integers $n$ such that $1<n<10^6$ and $n^3-1$ is divisible by $10^6 n-1$.
let $(a_{n})$ be an arbitrary sequence of positive numbers. Show that
$$\limsup_{n\to \infty} \left(\frac{a_1 +a_{n+1}}{a_{n}}\right)^{n} \geq e.$$
A city is laid out with a rectangular grid of roads with 10 streets numbered from 1 to 10 running east-west and 16 avenues numbered from 1 to 16 running northsouth. All streets end at First and Sixteenth Avenues, and all avenues end at First and Tenth Streets. A rectangular city park is bounded on the north and south by Sixth Street and Eighth Street, and bounded on the east and west by Fourth Avenue and Twelfth Avenue. Although there are no breaks in the roads that bound the park, no road goes through the park. The city paints a crosswalk from every street corner across any adjacent road. Thus, where two roads cross such as at Second Street and Second Avenue, there are four crosswalks painted, while at corners such as First Street and First Avenue, there are only two crosswalks painted.
How many crosswalks are there painted on the roads of this city?
A retiring employee receives and annual pension proportional to the square root of the number of years of his service. Had he served $a$ years more, his pension would have been $p$ dollars greater, whereas, had he served $b$ years more $b\neq a$, his pension would have been $q$ dollars greater than the original annual pension. Find his annual pension in terms of $a,b,p,$ and $q$.
$\textbf{(A) }\dfrac{p^2-q^2}{2(a-b)}\qquad\textbf{(B) }\dfrac{(p-q)^2}{2\sqrt{ab}}\qquad\textbf{(C) }\dfrac{ap^2-bq^2}{2(ap-bq)}\qquad\textbf{(D) }\dfrac{aq^2-bp^2}{2(bp-aq)}\qquad \textbf{(E) }\sqrt{(a-b)(p-q)}$
Let $f$ be an increasing mapping from the family of subsets of a given finite set $H$ into itself, i.e. such that for every $X \subseteq Y\subseteq H$ we have $f (X )\subseteq f (Y )\subseteq H .$ Prove that there exists a subset $H_{0}$ of $H$ such that $f (H_{0}) = H_{0}.$
Let $ABC$ be a triangle with $\angle B \ge 2\angle C$. Denote by $D$ the foot of the altitude from $A$ and by $M$ be the midpoint of $BC$. Prove that $DM \ge \frac{AB}{2}$.
There are $N$ students in a class. Each possible nonempty group of students selected a positive integer. All of these integers are distinct and add up to 2014. Compute the greatest possible value of $N$.
The vertices of a convex $n$-gon are colored so that adjacent vertices have different colors. Prove that if $n$ is odd, then the polygon can be divided into triangles with non-intersecting diagonals such that no diagonal has its endpoints the same color.
Let $\mathbb{R}[x]$ be the set of all polynomials with real coefficients. Find all functions $f: \mathbb{R}[x] \rightarrow \mathbb{R}[x]$ satisfying the following conditions:
[list]
[*] $f$ maps the zero polynomial to itself,
[*] for any non-zero polynomial $P \in \mathbb{R}[x]$, $\text{deg} \, f(P) \le 1+ \text{deg} \, P$, and
[*] for any two polynomials $P, Q \in \mathbb{R}[x]$, the polynomials $P-f(Q)$ and $Q-f(P)$ have the same set of real roots.
[/list]
[i]Proposed by Anant Mudgal, Sutanay Bhattacharya, Pulkit Sinha[/i]
Let $L$ be an arbitrary point on the minor arc $CD$ of the circumcircle of square $ABCD$. Let $K,M,N$ be the intersection points of $AL,CD$; $CL,AD$; and $MK,BC$ respectively. Prove that $B,M,L,N$ are concyclic.
A tourist traveling on a bus from the VIAZUL company arrived at the station in the city of Cienfuegos and immediately set out to take a walking tour of the city. After visiting the Terry theater in this city, decided to return to the station but walking only for blocks traveled an odd number of times in his previous walk. Can he do this on his return regardless. of the initial route?
Let $ a$, $ b$, $ c$ be the sides of a triangle, and $ S$ its area. Prove:
\[ a^{2} \plus{} b^{2} \plus{} c^{2}\geq 4S \sqrt {3}
\]
In what case does equality hold?
Let $a, b, c$ and $d$ be real numbers such that
$$a^3+b^3+c^3+d^3=a+b+c+d=0$$
Prove that the sum of a pair of these numbers is equal to $0$.
(LD Kurliandchik)
A sequence $(a_n)_{-\infty}^{-\infty}$ of zeros and ones is given. It is known that $a_n = 0$ if and only if $a_{n-6} + a_{n-5} +...+ a_{n-1}$ is a multiple of $3$, and not all terms of the sequence are zero. Determine the maximum possible number of zeros among $a_0,a_1,...,a_{97}$.
Let $P$ be a point and $\omega$ be a circle with center $O$ and radius $r$ . We define the power of the point $P$ with respect to the circle $\omega$ to be $OP^2 - r^2$ , and we denote this by pow $(P, \omega)$. We define the radical axis of two circles $\omega_1$ and $\omega_2$ to be the locus of all points P such that pow $(P,\omega_1) =$ pow $(P,\omega_2)$. It turns out that the pairwise radical axes of three circles are either concurrent or pairwise parallel. The concurrence point is referred to as the radical center of the three circles.
In $\vartriangle ABC$, let $I$ be the incenter, $\Gamma$ be the circumcircle, and $O$ be the circumcenter. Let $A_1,B_1,C_1$ be the point of tangency of the incircle of $\vartriangle ABC$ with side $BC,CA, AB$, respectively. Let $X_1,X_2 \in \Gamma$ such that $X_1,B_1,C_1,X_2$ are collinear in this order. Let $M_A$ be the midpoint of $BC$, and define $\omega_A$ as the circumcircle of $\vartriangle X_1X_2M_A$. Define $\omega_B$ ,$\omega_C$ analogously. The goal of this problem is to show that the radical center of $\omega_A$, $\omega_B$, $\omega_C$ lies on line $OI$.
(a) Let$ A'_1$ denote the intersection of $B_1C_1$ and $BC$. Show that $\frac{A_1B}{A_1C}=\frac{A'_1B}{A'_1C}$.
(b) Prove that $A_1$ lies on $\omega_A$.
(c) Prove that $A_1$ lies on the radical axis of $\omega_B$ and $\omega_C$ .
(d) Prove that the radical axis of $\omega_B$ and $\omega_C$ is perpendicular to $B_1C_1$.
(e) Prove that the radical center of $\omega_A$, $\omega_B$, $\omega_C$ is the orthocenter of $\vartriangle A_1B_1C_1$.
(f ) Conclude that the radical center of $\omega_A$, $\omega_B$, $\omega_C$ , $O$, and $I$ are collinear.
PS. You had better use hide for answers.
Through the midpoint $M$ of the side $BC$ of the triangle $ABC$ passes a line which intersects the rays $AB$ and $AC$ at $D$ and $E$, respectively, such that $AD=AE$. Let $F$ be the foot of the perpendicular from $A$ onto $BC$ and $P{}$ the circumcenter of triangle $ADE$. Prove that $PF=PM$.
On grid paper, mark three nodes so that in the triangle they formed, the sum of the two smallest medians equals to half-perimeter.
Let $x_1,x_2,\ldots x_n$ be positive integers such that no one of them is an initial fragment of any other (for example, $12$ is an initial fragment of $\underline{12},\underline{12}5$ and $\underline{12}405$). Prove that
\[\frac{1}{x_1}+\frac{1}{x_2}+\ldots+\frac{1}{x_n}<3. \]
Prove that, for all positive real numbers $ a$, $ b$, $ c$, the inequality
\[ \frac {1}{a^3 \plus{} b^3 \plus{} abc} \plus{} \frac {1}{b^3 \plus{} c^3 \plus{} abc} \plus{} \frac {1}{c^3 \plus{} a^3 \plus{} abc} \leq \frac {1}{abc}
\]
holds.
Find maximum value of number $a$ such that for any arrangement of numbers $1,2,\ldots ,10$ on a circle, we can find three consecutive numbers such their sum bigger or equal than $a$.
Show that there is no polyhedron whose projection on the plane is a nondegenerate triangle.