Found problems: 85335
For any set $S$, let $|S|$ denote the number of elements in $S$, and let $n(S)$ be the number of subsets of $S$, including the empty set and the set $S$ itself. If $A$, $B$ and $C$ are sets for which \[n(A) + n(B) + n(C) = n(A \cup B \cup C)\quad\text{and}\quad |A| = |B| = 100,\] then what is the minimum possible value of $|A \cap B \cap C|$?
$ \textbf{(A)}\ 96\qquad\textbf{(B)}\ 97\qquad\textbf{(C)}\ 98\qquad\textbf{(D)}\ 99\qquad\textbf{(E)}\ 100 $
Show that there exists an increasing sequence $a_1,a_2,a_3,...$ of natural numbers such that, for any integer $k \ge 2$, the sequence $k+a_n$ ($n \in N$) contains only finitely many primes.
Let $ABCD$ be a cyclic quadrilateral with $AB=4,BC=5,CD=6,$ and $DA=7$. Let $A_1$ and $C_1$ be the feet of the perpendiculars from $A$ and $C$, respectively, to line $BD,$ and let $B_1$ and $D_1$ be the feet of the perpendiculars from $B$ and $D,$ respectively, to line $AC$. The perimeter of $A_1B_1C_1D_1$ is $\frac mn$, where $m$ and $n$ are relatively prime positive integers. Find $m+n$.
Two people play a game on a $9 \times 9$ board. They move alternately. On each move, the first player draws a cross in an empty cell, and the second player draws a nought in an empty cell. When all $81$ cells are filled, the number $K$ of rows and columns in which there are more crosses and the number $H$ of rows and columns in which there are more noughts are counted. The score for the first player is the difference $B = K- H$. Find a value of $B$ such that the first player can guarantee a score of at least $B$, while the second player can hold the first player's score to at most B, regardless how the opponent plays.
(A Kanel)
Find all polynomials $P$ such that $P(x)+3P(x+2)=3P(x+1)+P(x+3)$ for all real numbers $x$.
Find all integers $n$ for which $n^7-41$ is the square of an integer
There are $100$ numbers on circle, and no one number is divided by other. In same time for all numbers we make next operation:
If $(a,b)$ are two neighbors ($a$ is left neighbor) , then we write between $a,b$ number $\frac{a}{(a,b)}$ and erase $a,b$
This operation was repeated some times. What maximum number of $1$ we can receive ?
Example: If we have circle with $3$ numbers $4,5,6$ then after operation we receive circle with numbers $\frac{4}{(4,5)}=4,\frac{5}{(5,6)}=5, \frac{6}{(6,4)}=3$.
[b]5.[/b] Let $\xi _{1},\xi _{2},\dots ,\xi _{n},... $ be independent random variables of uniform distribution in $(0,1)$. Show that the distribution of the random variable
$\eta _{n}= \sqrt[]{n}\prod_{k=1}^{n}(1-\frac{\xi _{k}}{k}) (n= 1,2,...)$
tends to a limit distribution for $n \to \infty $. [b](P. 6)[/b]
Given that $a, b, c, d \in \mathbb{R}$, prove that $(ab+cd)^2 \leq (a^2+c^2)(b^2+d^2)$.
Let $n>1$ be a positive integer. Show that the number of residues modulo $n^2$ of the elements of the set $\{ x^n + y^n : x,y \in \mathbb{N} \}$ is at most $\frac{n(n+1)}{2}$.
[I]Proposed by N. Safaei (Iran)[/i]
Let $\, ABCD \,$ be a convex quadrilateral such that diagonals $\, AC \,$ and $\, BD \,$ intersect at right angles, and let $\, E \,$ be their intersection. Prove that the reflections of $\, E \,$ across $\, AB, \, BC, \, CD, \, DA \,$ are concyclic.
A geometrical progression consists of $37$ positive integers. The first and the last terms are relatively prime numbers. Prove that the $19^{th}$ term of the progression is the $18^{th}$ power of some positive integer.
[i]($3$ points)[/i]
A cubic function $f(x)$ satisfies the equation $\sin 3t=f(\sin t)$ for all real numbers $t$.
Evaluate $\int_0^1 f(x)^2\sqrt{1-x^2}\ dx$.
The incircle of triangle $ABC$ with $AB\neq AC$ has center $I$ and is tangent to $BC, CA,$
and $AB$ at $D, E,$ and $F$ respectively. The circumcircle of triangle $ADI$ intersects $AB$ and
$AC$ again at $X$ and $Y.$ Prove that $EF$ bisects $XY.$
$a, a_1,a_2,\dots ,a_n$ are natural numbers. We know that for any natural number $k$ which $ak+1$ is square, at least one of $a_1k+1,\dots ,a_n k+1$ is also square.
Prove $a$ is one of $a_1,\dots ,a_n$
[i]Proposed by Mohsen Jamali[/i]
Solve the following equation in $\mathbb{R}$:
$$\left(x-\frac{1}{x}\right)^\frac{1}{2}+\left(1-\frac{1}{x}\right)^\frac{1}{2}=x.$$
Given rectangle $ R_1$ with one side $ 2$ inches and area $ 12$ square inches. Rectangle $ R_2$ with diagonal $ 15$ inches is similar to $ R_1.$ Expressed in square inches the area of $ R_2$ is:
$ \textbf{(A)}\ \frac92 \qquad
\textbf{(B)}\ 36 \qquad
\textbf{(C)}\ \frac{135}{2} \qquad
\textbf{(D)}\ 9 \sqrt{10} \qquad
\textbf{(E)}\ \frac{27 \sqrt{10}}{4}$
Consider a triangle $\vartriangle ABC$ with $BC = 10$. An excircle is a circle that is tangent to one side of the triangle as well as the extensions of the other two sides; suppose that the excircle opposite vertex $B$ has center $I_2$ and exradius $r_2 = 11$, and suppose that the excircle opposite vertex $C$ has center $I_3$ and exradius $r_3 = 13$. Compute $I_2I_3$.
Fix a positive integer $a > 1$. Consider triples $(f(x), g(x), h(x))$ of polynomials with integer coefficients, such that
1. $f$ is a monic polynomial with $\deg f \ge 1$.
2. There exists a positive integer $N$ such that $g(x)>0$ for $x \ge N$ and for all positive integers $n \ge N$, we have $f(n) \mid a^{g(n)} + h(n)$.
Find all such possible triples.
[i]Proposed by Mainak Ghosh and Rijul Saini[/i]
A point $P$ lies in the same plane as a given square of side $1$. Let the vertices of the square, taken counterclockwise, be $A$, $B$, $C$ and $D$. Also, let the distances from $P$ to $A$, $B$ and $C$, respectively, be $u$, $v$ and $w$. What is the greatest distance that $P$ can be from $D$ if $u^2 + v^2 = w^2$?
$ \textbf{(A)}\ 1 + \sqrt{2}\qquad\textbf{(B)}\ 2\sqrt{2}\qquad\textbf{(C)}\ 2 + \sqrt{2}\qquad\textbf{(D)}\ 3\sqrt{2}\qquad\textbf{(E)}\ 3 + \sqrt{2}$
Prove that if the positive real numbers $a, b, c$ satisfy the equation
\[a^4 + b^4 + c^4 + 4a^2b^2c^2 = 2 (a^2b^2 + a^2c^2 + b^2c^2),\]
then there is a triangle $ABC$ with internal angles $\alpha, \beta, \gamma$ such that
\[\sin \alpha = a, \qquad \sin \beta = b, \qquad \sin \gamma= c.\]
Determine all pairs $(x,y)$ of positive integers satisfying the equation \[(x+y)^{2}-2(xy)^{2}=1.\]
In a triangle $ABC$, the points $A’$, $B’$ and $C’$ on the sides opposite $A$ ,$B$ and $C$, respectively, are such that the lines $AA’$, $BB’$ and $CC’$ are concurrent. Prove that the diameter of the circumscribed circle of the triangle $ABC$ equals the product $|AB’|\cdot |BC’|\cdot |CA’|$ divided by the area of the triangle $A’B’C’$.
A railway line is divided into ten sections by the stations $A,B,C,D,E,F, G,H,I,J,K$. The length of each section is an integer number of kilometers and the distacne between $A$ and $K$ is $56$ km. A trip along two successive sections never exceeds $12$ km, but a trip along three successive sections is at least $17$ km. What is the distance between $B$ and $G$?
[img]https://cdn.artofproblemsolving.com/attachments/1/f/202ddf633ed6da8692bf4d0b1fc0af59548526.png[/img]
Positive integers $k, m, n ,p $ integers are such that $p=2^{2^n}+1$ is prime and $p\mid 2^k-m$. Prove that there exists a positive integer $l$ such that $p^2\mid 2^l-m$.