Found problems: 15925
1976 Poland - Second Round, 5
Prove that if $ \cos \pi x =\frac{1}{3} $ then $ x $ is an irrational number.
1971 Poland - Second Round, 6
Given an infinite sequence $ \{a_n\} $. Prove that if
$$ a_n + a_{n+2} > 2a_{n+1} \ \ for \ \ n = 1, 2 ... $$
then $$ \frac{a_1+a_3+\ldots a_{2n+1}}{n+1} \geq \frac{a_2+a_4+\ldots a_{2n}}{n} $$
for $ n = 1, 2, \ldots $.
2006 Estonia Math Open Senior Contests, 2
After the schoolday is over, Juku must attend an extra math class. The teacher
writes a quadratic equation $ x^2\plus{} p_1x\plus{}q_1 \equal{} 0$ with integer coefficients on the blackboard and Juku has to find its solutions. If they are not both integers, Jukumay go home. If the solutions are integers, then the teacher writes a new equation $ x^2 \plus{} p_2x \plus{} q_2 \equal{} 0,$ where $ p_2$ and $ q_2$ are the solutions of the previous equation taken in some order, and everything starts all over. Find all possible values for $ p_1$ and $ q_1$ such that the teacher can hold Juku at school forever.
2019 District Olympiad, 1
Determine the numbers $x,y$, with $x$ integer and $y$ rational, for which equality holds:
$$5(x^2+xy+y^2) = 7(x+2y)$$
2011 All-Russian Olympiad Regional Round, 10.4
Non-zero real numbers $a$, $b$ and $c$ are such that any two of the three equations $ax^{11}+bx^4+c=0$, $bx^{11}+cx^4+a=0$, $cx^{11}+ax^4+b=0$ have a common root. Prove that all three equations have a common root. (Author: I. Bogdanov)
2006 India Regional Mathematical Olympiad, 3
If $ a,b,c$ are three positive real numbers, prove that $ \frac {a^{2}\plus{}1}{b\plus{}c}\plus{}\frac {b^{2}\plus{}1}{c\plus{}a}\plus{}\frac {c^{2}\plus{}1}{a\plus{}b}\ge 3$
2016 Belarus Team Selection Test, 2
Find all real numbers $a$ such that exists function $\mathbb {R} \rightarrow \mathbb {R} $ satisfying the following conditions:
1) $f(f(x)) =xf(x)-ax$ for all real $x$
2) $f$ is not constant
3) $f$ takes the value $a$
2012 ELMO Shortlist, 7
Let $f,g$ be polynomials with complex coefficients such that $\gcd(\deg f,\deg g)=1$. Suppose that there exist polynomials $P(x,y)$ and $Q(x,y)$ with complex coefficients such that $f(x)+g(y)=P(x,y)Q(x,y)$. Show that one of $P$ and $Q$ must be constant.
[i]Victor Wang.[/i]
1935 Moscow Mathematical Olympiad, 001
Find the ratio of two numbers if the ratio of their arithmetic mean to their geometric mean is $25 : 24$
1992 Baltic Way, 3
Find an infinite non-constant arithmetic progression of natural numbers such that each term is neither a sum of two squares, nor a sum of two cubes (of natural numbers).
2017 Ecuador NMO (OMEC), 3
Adrian has $2n$ cards numbered from $ 1$ to $2n$. He gets rid of $n$ cards that are consecutively numbered. The sum of the numbers of the remaining papers is $1615$. Find all the possible values of $n$.
2016 BMT Spring, 8
Evaluate the following limit
$$\lim_{x\to 0} (1 + 2x + 3x^2 + 4x^3 +...)^{1/x}$$
2021 IMO Shortlist, A2
Which positive integers $n$ make the equation \[\sum_{i=1}^n \sum_{j=1}^n \left\lfloor \frac{ij}{n+1} \right\rfloor=\frac{n^2(n-1)}{4}\] true?
2024 APMO, 3
Let $n$ be a positive integer and let $a_1, a_2, \ldots, a_n$ be positive reals. Show that $$\sum_{i=1}^{n} \frac{1}{2^i}(\frac{2}{1+a_i})^{2^i} \geq \frac{2}{1+a_1a_2\ldots a_n}-\frac{1}{2^n}.$$
1982 Austrian-Polish Competition, 5
Show that [0,1] cannot be partitioned into two disjoints sets A and B such that B=A+a for some real a.
2010 Gheorghe Vranceanu, 4
Let be two real numbers $ \alpha ,\beta $ and two sequences $ \left(x_n \right)_{n\ge 1} ,\left(y_n \right)_{n\ge 1} $ whose smallest periods are $ p,q, $ respectively. Prove that the sequence $ \left( \alpha x_n+\beta y_n\right)_{n\ge 1} $ is periodic if $ \text{gcd}^2 (p,q) | \text{lcm} (p,q) , $ and in this case find its smallest period.
LMT Accuracy Rounds, 2023 S Tie
Estimate the value of $$\sum^{2023}_{n=1} \left(1+ \frac{1}{n} \right)^n$$ to $3$ decimal places.
2023 May Olympiad, 1
At Easter Day, $4$ children and their mothers participated in a game in which they had to find hidden chocolate eggs. Augustine found $4$ eggs, Bruno found $6$, Carlos found $9$ and Daniel found $12$. Mrs. Gómez found the same number of eggs as her son, Mrs. Junco found twice as many eggs as her son, Mrs. Messi found three times as many eggs as her son, and Mrs. Núñez found five times as many eggs as her son. At the end of the day, they put all the eggs in boxes, with $18$ eggs in each box, and only one egg was left over. Determine who the mother of each child is.
1990 Rioplatense Mathematical Olympiad, Level 3, 1
How many positive integer solutions does the equation have $$\left\lfloor\frac{x}{10}\right\rfloor= \left\lfloor\frac{x}{11}\right\rfloor + 1?$$
($\lfloor x \rfloor$ denotes the integer part of $x$, for example $\lfloor 2\rfloor = 2$, $\lfloor \pi\rfloor = 3$, $\lfloor \sqrt2 \rfloor =1$)
1976 All Soviet Union Mathematical Olympiad, 223
The natural numbers $x_1$ and $x_2$ are less than $1000$. We construct a sequence:
$$x_3 = |x_1 - x_2|$$
$$x_4 = min \{ |x_1 - x_2|, |x_1 - x_3|, |x_2 - x_3|\}$$
$$...$$
$$x_k = min \{ |x_i - x_j|, 0 <i < j < k\}$$
$$...$$
Prove that $x_{21} = 0$.
2000 All-Russian Olympiad Regional Round, 11.1
Prove that it is possible to choose different real numbers $a_1, a_2, . . . , a_{10}$ that the equation $$(x - a_1)(x -a_2).... (x -a_{10}) = (x + a_1)(x + a_2) ...(x + a_{10})$$ will have exactly $5$ different real roots.
2003 IMO Shortlist, 3
Consider pairs of the sequences of positive real numbers \[a_1\geq a_2\geq a_3\geq\cdots,\qquad b_1\geq b_2\geq b_3\geq\cdots\] and the sums \[A_n = a_1 + \cdots + a_n,\quad B_n = b_1 + \cdots + b_n;\qquad n = 1,2,\ldots.\] For any pair define $c_n = \min\{a_i,b_i\}$ and $C_n = c_1 + \cdots + c_n$, $n=1,2,\ldots$.
(1) Does there exist a pair $(a_i)_{i\geq 1}$, $(b_i)_{i\geq 1}$ such that the sequences $(A_n)_{n\geq 1}$ and $(B_n)_{n\geq 1}$ are unbounded while the sequence $(C_n)_{n\geq 1}$ is bounded?
(2) Does the answer to question (1) change by assuming additionally that $b_i = 1/i$, $i=1,2,\ldots$?
Justify your answer.
2011 All-Russian Olympiad, 1
A quadratic trinomial $P(x)$ with the $x^2$ coefficient of one is such, that $P(x)$ and $P(P(P(x)))$ share a root. Prove that $P(0)*P(1)=0$.
2016 China Northern MO, 5
$a_1=2,a_{n+1}=\frac{2^{n+1}a_n}{(n+\frac{1}{2})a_n+2^n}(n\in\mathbb{Z}_+)$
[b](a)[/b] Find $a_n$.
[b](b)[/b] Let $b_n=\frac{n^3+2n^2+2n+2}{n(n+1)(n^2+1)a_n}$.
Find $S_n=\sum_{i=1}^nb_i$.
2011 NIMO Problems, 9
The roots of the polynomial $P(x) = x^3 + 5x + 4$ are $r$, $s$, and $t$. Evaluate $(r+s)^4 (s+t)^4 (t+r)^4$.
[i]Proposed by Eugene Chen
[/i]