## Posts

Showing posts with the label algebra

### 2003 IMO SL #A6

Let $n$ be a positive integer and let $(x_1,\ldots,x_n)$, $(y_1,\ldots,y_n)$ be two sequences of positive real numbers. Suppose $(z_2,\ldots,z_{2n})$ is a sequence of positive real numbers such that $z_{i+j}^2 \geq x_iy_j$ for all $1\le i,j \leq n$. Let $M=\max\{z_2,\ldots,z_{2n}\}$. Prove that$\left( \frac{M+z_2+\dots+z_{2n}}{2n} \right)^2 \ge \left( \frac{x_1+\dots+x_n}{n} \right) \left( \frac{y_1+\dots+y_n}{n} \right).$

### 2007 IMO SL #A6

Let $a_1, a_2, \ldots, a_{100}$ be nonnegative real numbers such that $a^2_1 + a^2_2 + \ldots + a^2_{100} = 1.$ Prove that $a^2_1 \cdot a_2 + a^2_2 \cdot a_3 + \ldots + a^2_{100} \cdot a_1 < \frac {12}{25}.$

### 2018 IMO SL #A1

Let $\mathbb{Q}_{>0}$ denote the set of all positive rational numbers. Determine all functions $f:\mathbb{Q}_{>0}\to \mathbb{Q}_{>0}$ satisfying$$f(x^2f(y)^2)=f(x)^2f(y)$$for all $x,y\in\mathbb{Q}_{>0}$

### 2017 IMO SL #A1

Let $a_1,a_2,\ldots a_n,k$, and $M$ be positive integers such that $$\frac{1}{a_1}+\frac{1}{a_2}+\cdots+\frac{1}{a_n}=k\quad\text{and}\quad a_1a_2\cdots a_n=M.$$If $M>1$, prove that the polynomial $$P(x)=M(x+1)^k-(x+a_1)(x+a_2)\cdots (x+a_n)$$has no positive roots.

### 2016 India IMO Training Camp #3

Let $a,b,c,d$ be real numbers satisfying $|a|,|b|,|c|,|d|>1$ and $abc+abd+acd+bcd+a+b+c+d=0$. Prove that $\frac {1} {a-1}+\frac {1} {b-1}+ \frac {1} {c-1}+ \frac {1} {d-1} >0$

### 2007 IMO Sl #A1

Real numbers $a_{1}$, $a_{2}$, $\ldots$, $a_{n}$ are given. For each $i$, $(1 \leq i \leq n )$, define $d_{i} = \max \{ a_{j}\mid 1 \leq j \leq i \} - \min \{ a_{j}\mid i \leq j \leq n \}$ and let $d = \max \{d_{i}\mid 1 \leq i \leq n \}$.  (a) Prove that, for any real numbers $x_{1}\leq x_{2}\leq \cdots \leq x_{n}$, $\max \{ |x_{i} - a_{i}| \mid 1 \leq i \leq n \}\geq \frac {d}{2}. \quad \quad (*)$  (b) Show that there are real numbers $x_{1}\leq x_{2}\leq \cdots \leq x_{n}$ such that the equality holds in (*).

### 2000 IMO #2

Let $a, b, c$ be positive real numbers so that $abc = 1$. Prove that $\left( a - 1 + \frac 1b \right) \left( b - 1 + \frac 1c \right) \left( c - 1 + \frac 1a \right) \leq 1.$

### 2003 USA TST #4

Let $\mathbb{N}$ denote the set of positive integers. Find all functions $f: \mathbb{N} \to \mathbb{N}$ such that$f(m+n)f(m-n) = f(m^2)$for $m,n \in \mathbb{N}$.

### 2020 International Zhautykov Olympiad #3

Given convex hexagon $ABCDEF$, inscribed in the circle. Prove that $$AC*BD*DE*CE*EA*FB \geq 27 AB * BC * CD * DE * EF * FA$$

### 2018 Kazakhstan MO #4

Prove that for all reals $a,b,c,d\in(0,1)$ we have$$\left(ab-cd\right)\left(ac+bd\right)\left(ad-bc\right)+\min{\left(a,b,c,d\right)} < 1.$$

### 2021 Mediterranean MO #1

Determine the smallest positive integer $M$ with the following property: For every choice of integers $a,b,c$, there exists a polynomial $P(x)$ with integer coefficients so that $P(1)=aM$ and $P(2)=bM$ and $P(4)=cM$.

### 2010 IMO SL #A5

Denote by $\mathbb{Q}^+$ the set of all positive rational numbers. Determine all functions $f : \mathbb{Q}^+ \mapsto \mathbb{Q}^+$ which satisfy the following equation for all $x, y \in \mathbb{Q}^+:$$f\left( f(x)^2y \right) = x^3 f(xy).$

### 2018 JBMO SL #A6

For $a,b,c$ positive real numbers such that $ab+bc+ca=3$, prove $$\frac{a}{\sqrt{a^3+5}}+\frac{b}{\sqrt{b^3+5}}+\frac{c}{\sqrt{c^3+5}} \leq \frac{\sqrt{6}}{2}$$

### 2013 BAMO-8 #4

For a positive integer $n>2$, consider the $n-1$ fractions$$\dfrac21, \dfrac32, \cdots, \dfrac{n}{n-1}$$The product of these fractions equals $n$, but if you reciprocate (i.e. turn upside down) some of the fractions, the product will change. Can you make the product equal 1? Find all values of $n$ for which this is possible and prove that you have found them all.

### 2009 JBMO Shortlist #A1

Determine all integers $a, b, c$ satisfying the identities $$a + b + c = 15$$  $$(a - 3)^3 + (b - 5)^3 + (c -7)^3 = 540.$$

### 2013 APMO #2

For $2k$ real numbers $a_1, a_2, ..., a_k$, $b_1, b_2, ..., b_k$ define a sequence of numbers $X_n$ by $X_n = \sum_{i=1}^k [a_in + b_i] \quad (n=1,2,...).$If the sequence $X_N$ forms an arithmetic progression, show that $\textstyle\sum_{i=1}^k a_i$ must be an integer. Here $[r]$ denotes the greatest integer less than or equal to $r$.

### 2011 China National Olympiad #5

Let $a_i,b_i,i=1,\cdots,n$ are nonnegative numbers,and $n\ge 4$,such that $a_1+a_2+\cdots+a_n=b_1+b_2+\cdots+b_n>0$. Find the maximum of $\frac{\sum_{i=1}^n a_i(a_i+b_i)}{\sum_{i=1}^n b_i(a_i+b_i)}$

### 2018 CMI Entrance Exam #2

$\textbf{(a)}$ Find all real solutions of the equation$$\Big(x^2-2x\Big)^{x^2+x-6}=1$$Explain why your solutions are the only solutions. $\textbf{(b)}$ The following expression is a rational number. Find its value.$$\sqrt{6\sqrt{3}+10} -\sqrt{6\sqrt{3}-10}$$

Given a strictly increasing infinite sequence of natural numbers $a_1,$ $a_2,$ $a_3,$ $\ldots$. It is known that $a_n \leq n + 2020$ and the number $n ^ 3 a_n - 1$ is divisible by $a_ {n + 1}$ for all natural numbers $n$. Prove that $a_n = n$ for all natural numbers $n$.
Given that ${a_n}$ and ${b_n}$ are two sequences of integers defined by $$a_1=1, a_2=10, a_{n+1}=2a_n+3a_{n-1}, \dots \text{for }n=2,3,4,\ldots,$$ $$b_1=1, b_2=8, b_{n+1}=3b_n+4b_{n-1}, \dots \text{for }n=2,3,4,\ldots.$$ Prove that, besides the number $1$, no two numbers in the sequences are identical.