Found problems: 85335
Let $\tau(n)$ denote the number of positive divisors of the positive integer $n$. Prove that there exist infinitely many positive integers $a$ such that the equation $ \tau(an)=n $ does not have a positive integer solution $n$.
Let $a$, $b$, $c$ be positive real numbers such that $a + b + c = 1$. Prove that $$\sqrt[3]{\left (\frac{1+a}{b+c}\right )^{\frac{1-a}{bc}}\left (\frac{1+b}{c+a}\right )^{\frac{1-b}{ca}}\left (\frac{1+c}{a+b}\right )^{\frac{1-c}{ab}}} \geq 64 $$
Given is a grid with $2$ rows and $120$ columns, such that each cell has a number from the set $1, 2, ..., 120$. It is known that in each column, the upper number in it is smaller than the lower number, and in each row, the numbers are in non-strict increasing order from left to right. Prove that the number of these tables is multiple of $239$.
Let's call a natural number a palindrome, the decimal notation of which is equally readable from left to right and right to left (decimal notation cannot start from zero; for example, the number $1221$ is a palindrome, but the numbers $1231, 1212$ and $1010$ are not). Which palindromes among the numbers from $10,000$ to $999,999$ have an odd sum of digits, which have an one even, and how many times are the ones with odd sum more than the ones with the even sum?
Let $a_1,a_2, a_3,...$ be a sequence of real numbers which satisfy the relation $a_{n+1} =\sqrt{a_n^2 + 1}$
Suppose that there exists a positive integer $n_0$ such that $a_{2n_0} = 3a_{n_0}$ . Find the value of $a_{46}$.
Decide if it is possible to choose $330$ points in the plane so that among all the distances that are formed between two of them there are at least $1700$ that are equal.
Let triangle $\vartriangle ABC$ be a triangle with $AB = 5$, $BC = 7$, and $CA = 8$, and let $I$ be the incenter of $\vartriangle ABC$. Let circle $C_A$ denote the circle with center $A$ and radius $\overline{AI}$, denote $C_B$ and circle $C_C$ similarly. Besides all intersecting at $I$, the circles $C_A$,$C_B$,$C_C$ also intersect pairwise at $F$, $G$, and $H$. Compute the area of triangle $\vartriangle FGH$.
Given is triangle $ABC$ with an inscribed circle with center $M$ and radius $r$.
The tangent to this circle parallel to $BC$ intersects $AC$ in $D$ and $AB$ in $E$.
The tangent to this circle parallel to $AC$ intersects $AB$ in $F$ and $BC$ in $G$.
The tangent to this circle parallel to $AB$ intersects $BC$ in $H$ and $AC$ in $K$.
Name the centers of the inscribed circles of triangle $AED$, triangle $FBG$ and triangle $KHC$ successively $M_A, M_B, M_C$ and the rays successively $r_A, r_B$ and $r_C$.
Prove that $r_A + r_B + r_C = r$.
Prove that for every $ \vartheta$, $ 0<\vartheta<1$, there exist a sequence $ \lambda_n$ of positive integers and a series $ \sum_{n=1}^{\infty} a_n$ such that
(i)$ \lambda_{n+1}-\lambda_n > (\lambda_n)^{\vartheta}$,
(ii) $ \lim_{r\rightarrow 1^-} \sum_{n=1}^{\infty} a_nr^{\lambda_n}$ exists,
(iii) $ \sum _{n=1}^{\infty} a_n$ is divergent.
[i]P. Turan[/i]
Find all triplets of integers $(x,y,z)$ such that $$xy(x^2-y^2)+yz(y^2-z^2)+zx(z^2-x^2)=1$$
How many normals to an ellipse can be drawn from a given point in plane? Find the region in which the number of normals is maximal.
The Fibonacci sequence is the list of numbers that begins $1, 2, 3, 5, 8, 13$ and continues with each subsequent number being the sum of the previous two.
Prove that for every positive integer $n$ when the first $n$ elements of the Fibonacci sequence are alternately added and subtracted, the result is an element of the sequence or the negative of an element of the sequence.
For example, when $n = 4$ we have $1-2+3-5 = -3$ and $3$ is an element of the Fibonacci sequence.
What is the sum of the last two digits of $7^{42} + 7^{43}$ in base $10$.
$\text{(A) }2\qquad\text{(B) }3\qquad\text{(C) }8\qquad\text{(D) }9\qquad\text{(E) }11$
The SMO camp has at least four leaders. Any two leaders are either mutual friends or enemies. In every group of four leaders there is at least one who is with the three is friends with others. Is there always one leader who is friends with everyone else?
Prove that no number of the form $10^{-n}$, $n\geq 1,$ can be represented as the sum of reciprocals of factorials of different positive integers.
Find the sum of all real numbers $x$ such that $5x^4-10x^3+10x^2-5x-11=0$.
Andrew chooses three (not necessarily distinct) integers $a$, $b$, and $c$ independently and uniformly at random from $\{1,2,3,4,5,6,7\}$. Let $p$ be the probability that $abc(a+b+c)$ is divisible by $4$. If $p$ can be written as $\frac{m}{n}$ for relatively prime positive integers $m$ and $n$, then compute $m+n$.
[i]Proposed by Andrew Wen[/i]
The nine horizontal and nine vertical lines on an $8\times8$ checkerboard form $r$ rectangles, of which $s$ are squares. The number $s/r$ can be written in the form $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m+n.$
p1. Four kite-shaped shapes as shown below ($a > b$, $a$ and $b$ are natural numbers less than $10$) arranged in such a way so that it forms a square with holes in the middle. The square hole in the middle has a perimeter of $16$ units of length. What is the possible perimeter of the outermost square formed if it is also known that $a$ and $b$ are numbers coprime?
[img]https://cdn.artofproblemsolving.com/attachments/4/1/fa95f5f557aa0ca5afb9584d5cee74743dcb10.png[/img]
p2. If $a = 3^p$, $b = 3^q$, $c = 3^r$, and $d = 3^s$ and if $p, q, r$, and $s$ are natural numbers, what is the smallest value of $p\cdot q\cdot r\cdot s$ that satisfies $a^2 + b^3 + c^5 = d^7$
3. Ucok intends to compose a key code (password) consisting of 8 numbers and meet the following conditions:
i. The numbers used are $1, 2, 3, 4, 5, 6, 7, 8$, and $9$.
ii. The first number used is at least $1$, the second number is at least $2$, third digit-at least $3$, and so on.
iii. The same number can be used multiple times.
a) How many different passwords can Ucok compose?
b) How many different passwords can Ucok make, if provision (iii) is replaced with: no numbers may be used more than once.
p 4. For any integer $a, b$, and $c$ applies $a\times (b + c) = (a\times b) + (a\times c)$.
a) Look for examples that show that $a + (b\times c)\ne (a + b)\times (a + c)$.
b) Is it always true that $a + (b\times c) = (a + b)\times(a + c)$? Justify your answer.
p5. The results of a survey of $N$ people with the question whether they maintain dogs, birds, or cats at home are as follows: $50$ people keep birds, $61$ people don't have dogs, $13$ people don't keep a cat, and there are at least $74$ people who keep the most a little two kinds of animals in the house. What is the maximum value and minimum of possible value of $N$ ?
Let $ x, y, z $ be positive real numbers. Prove that
\[ \frac{2x^2 + xy}{(y+ \sqrt{zx} + z )^2} + \frac{2y^2 + yz}{(z+ \sqrt{xy} + x )^2} + \frac{2z^2 + zx}{(x+ \sqrt{yz} +y )^2} \ge 1 \]
Let be two natural primes $ 1\le q \le p. $ Prove that $ \left( \sqrt{p^2+q} +p\right)^2 $ is irrational and its fractional part surpasses $ 3/4. $
It is known that the real function $f(t)$ is monotonic increasing in the interval $-8 \le t \le 8$, but nothing is known about what happens outside of it. In what range of values of $x$, can it be ensured that the function $y = f(2x - x^2)$ is monotonic increasing?
Given a triangle with sides $a, b, c$ and medians $s_a, s_b, s_c$ respectively. Prove the following inequality:
$$a + b + c> s_a + s_b + s_c> \frac34 (a + b + c) $$
Let $ f(x) \equal{} \frac{x}{1 \minus{} x}$ and let $ a$ be a real number. If $ x_0 \equal{} a, x_1 \equal{} f(x_0), x_2 \equal{} f(x_1), ...., x_{1996} \equal{} f(x_{1995})$ and $ x_{1996} \equal{} 1,$ what is $ a$?
A. 0
B. 1/1997
C. 1995
D. 1995/1996
E. None of these
Define $C(\alpha)$ to be the coefficient of $x^{1992}$ in the power series about $x = 0$ of $(1 + x)^{\alpha}$ . Evaluate
$$\int_{0}^{1} \left( C(-y-1) \sum_{k=1}^{1992} \frac{1}{y+k} \right)\, dy.$$