Olympiad problems

2018 Romania National Olympiad, 2

Let $a, b, c, d$ be natural numbers such that $a + b + c + d = 2018$. Find the minimum value of the expression: $$E = (a-b)^2 + 2(a-c)^2 + 3(a-d)^2+4(b-c)^2 + 5(b-d)^2 + 6(c-d)^2.$$

2019 India IMO Training Camp, P1

Tags: inequalities , number theory

Let $a_1,a_2,\ldots, a_m$ be a set of $m$ distinct positive even numbers and $b_1,b_2,\ldots,b_n$ be a set of $n$ distinct positive odd numbers such that \[a_1+a_2+\cdots+a_m+b_1+b_2+\cdots+b_n=2019\] Prove that \[5m+12n\le 581.\]

2002 Federal Math Competition of S&M, Problem 1

Tags: inequalities

Real numbers $x,y,z$ satisfy the inequalities $$x^2\le y+z,\qquad y^2\le z+x\qquad z^2\le x+y.$$Find the minimum and maximum possible values of $z$.

2009 Indonesia TST, 4

Tags: inequalities

Let $ a$, $ b$, and $ c$ be positive real numbers such that $ ab + bc + ca = 3$. Prove the inequality \[ 3 + \sum_{\mathrm{\cyc}} (a - b)^2 \ge \frac {a + b^2c^2}{b + c} + \frac {b + c^2a^2}{c + a} + \frac {c + a^2b^2}{a + b} \ge 3. \]

2019 Jozsef Wildt International Math Competition, W. 25

Tags: function , inequalities , gamma function , summation

Let $x_i$, $y_i$, $z_i$, $w_i \in \mathbb{R}^+, i = 1, 2,\cdots n$, such that$$\sum \limits_{i=1}^nx_i=nx,\ \sum \limits_{i=1}^ny_i=ny,\ \sum \limits_{i=1}^nw_i=nw $$ $$\Gamma \left(z_i\right)\geq \Gamma \left(w_i\right),\ \sum \limits_{i=1}^n\Gamma \left(z_i\right)=n\Gamma^* (z)$$Then$$\sum \limits_{i=1}^n \frac{\left(\Gamma \left(x_i\right)+\Gamma \left(y_i\right)\right)^2}{\Gamma \left(z_i\right)-\Gamma \left(w_i\right)}\geq n\frac{\left(\Gamma \left(x\right)+\Gamma \left(y\right)\right)^2}{\Gamma^* \left(z\right)-\Gamma \left(w\right)}$$

1997 India Regional Mathematical Olympiad, 3

Tags: inequalities

Solve for real $x$: \[ \frac{1}{[x]} + \frac{1}{[2x]} = x - [x] + \frac{1}{3}. \]

2024 Stars of Mathematics, P1

Tags: floor function , inequalities

Fix a positive integer $n\geq 2$. What is the lest value that the expression $$\bigg\lfloor\frac{x_2+x_3+\dots +x_n}{x_1}\bigg\rfloor + \bigg\lfloor\frac{x_1+x_3+\dots +x_n}{x_2}\bigg\rfloor +\dots +\bigg\lfloor\frac{x_1+x_2+\dots +x_{n-1}}{x_n}\bigg\rfloor$$ may achieve, where $x_1,x_2,\dots ,x_n$ are positive real numbers.

1997 Turkey MO (2nd round), 1

Tags: inequalities

Let $e > 0$ be a given real number. Find the least value of $f(e)$ (in terms of $e$ only) such that the inequality $a^{3}+ b^{3}+ c^{3}+ d^{3} \leq e^{2}(a^{2}+b^{2}+c^{2}+d^{2}) + f(e)(a^{4}+b^{4}+c^{4}+d^{4})$ holds for all real numbers $a, b, c, d$.

2010 Czech And Slovak Olympiad III A, 6

Tags: least common multiple , inequalities , lcm , minimum value , minimum , number theory

Find the minimum of the expression $\frac{a + b + c}{2} -\frac{[a, b] + [b, c] + [c, a]}{a + b + c}$ where the variables $a, b, c$ are any integers greater than $1$ and $[x, y]$ denotes the least common multiple of numbers $x, y$.

2011 Morocco National Olympiad, 1

Tags: inequalities

Prove that \[2010< \frac{2^{2}+1}{2^{2}-1}+\frac{3^{2}+1}{3^{2}-1}+...+\frac{2010^{2}+1}{2010^{2}-1}< 2010+\frac{1}{2}.\]

2008 Serbia National Math Olympiad, 6

Tags: trigonometry , inequalities , geometry , cyclic quadrilateral

In a convex pentagon $ ABCDE$, let $ \angle EAB \equal{} \angle ABC \equal{} 120^{\circ}$, $ \angle ADB \equal{} 30^{\circ}$ and $ \angle CDE \equal{} 60^{\circ}$. Let $ AB \equal{} 1$. Prove that the area of the pentagon is less than $ \sqrt {3}$.

1996 French Mathematical Olympiad, Problem 4

Tags: inequality , optimization , inequalities

(a) A function $f$ is defined by $f(x)=x^x$ for all $x>0$. Find the minimum value of $f$. (b) If $x$ and $y$ are two positive real numbers, show that $x^y+y^x>1$.

VI Soros Olympiad 1999 - 2000 (Russia), 10.1

Tags: inequalities , algebra

For real numbers $x,y, \in [1,2]$, prove the inequality $3(x + y)\ge 2xy + 4$

1998 Romania National Olympiad, 2

Tags: algebra , complex number , inequalities

Let $a \ge1$ be a real number and $z$ be a complex number such that $| z + a | \le a$ and $|z^2+ a | \le a$. Show that $| z | \le a$.

2011 Mathcenter Contest + Longlist, 10

Tags: algebra , inequalities

Let $p,q,r\in R $ with $pqr=1$. Prove that $$\left(\frac{1}{1-p}\right)^2+\left(\frac{1}{1-q}\right)^2+\left(\frac{1}{1-r}\right)^2\ge 1$$ [i](Real Matrik)[/i]

2007 Hanoi Open Mathematics Competitions, 10

Tags: algebra , inequalities , minimum

What is the smallest possible value of $x^2+2y^2-x-2y-xy$?

2017 China Second Round Olympiad, 2

Tags: inequalities , algebra

Let $ x,y$ are real numbers such that $x^2+2cosy=1$. Find the ranges of $x-cosy$.

2006 AIME Problems, 15

Tags: function , absolute value , inequalities

Given that a sequence satisfies $x_0=0$ and $|x_k|=|x_{k-1}+3|$ for all integers $k\ge 1,$ find the minimum possible value of $|x_1+x_2+\cdots+x_{2006}|$.

2023 Tuymaada Olympiad, 6

Tags: plane , geometry inequality , inequalities , geometry

In the plane $n$ segments with lengths $a_1, a_2, \dots , a_n$ are drawn. Every ray beginning at the point $O$ meets at least one of the segments. Let $h_i$ be the distance from $O$ to the $i$-th segment (not the line!) Prove the inequality \[\frac{a_1}{h_1}+\frac{a_2}{h_2} + \ldots + \frac{a_i}{h_i} \geqslant 2 \pi.\]

2010 Korea National Olympiad, 2

Tags: inequalities

Let $ a, b, c $ be positive real numbers such that $ ab+bc+ca=1 $. Prove that \[ \sqrt{ a^2 + b^2 + \frac{1}{c^2}} + \sqrt{ b^2 + c^2 + \frac{1}{a^2}} + \sqrt{ c^2 + a^2 + \frac{1}{b^2}} \ge \sqrt{33} \]

2005 China Girls Math Olympiad, 5

Tags: inequalities

Let $ x$ and $ y$ be positive real numbers with $ x^3 \plus{} y^3 \equal{} x \minus{} y.$ Prove that \[ x^2 \plus{} 4y^2 < 1.\]

2002 China Team Selection Test, 2

Tags: reflection , incenter , inequalities , parallelogram , circumcircle , geometric transformation , geometry

Circles $ \omega_{1}$ and $ \omega_{2}$ intersect at points $ A$ and $ B.$ Points $ C$ and $ D$ are on circles $ \omega_{1}$ and $ \omega_{2},$ respectively, such that lines $ AC$ and $ AD$ are tangent to circles $ \omega_{2}$ and $ \omega_{1},$ respectively. Let $ I_{1}$ and $ I_{2}$ be the incenters of triangles $ ABC$ and $ ABD,$ respectively. Segments $ I_{1}I_{2}$ and $ AB$ intersect at $ E$. Prove that: $ \frac {1}{AE} \equal{} \frac {1}{AC} \plus{} \frac {1}{AD}$

2005 National Olympiad First Round, 7

Tags: rearrangement inequality , inequalities , trigonometry

What is the greatest value of $\sin x \cos y + \sin y \cos z + \sin z \cos x$, where $x,y,z$ are real numbers? $ \textbf{(A)}\ \sqrt 2 \qquad\textbf{(B)}\ \dfrac 32 \qquad\textbf{(C)}\ \dfrac {\sqrt 3}2 \qquad\textbf{(D)}\ 2 \qquad\textbf{(E)}\ 3 $

2006 National Olympiad First Round, 16

Tags: inequalities

How many positive integer tuples $ (x_1,x_2,\dots, x_{13})$ are there satisfying the inequality $x_1+x_2+\dots + x_{13}\leq 2006$? $ \textbf{(A)}\ \frac{2006!}{13!1993!} \qquad\textbf{(B)}\ \frac{2006!}{14!1992!} \qquad\textbf{(C)}\ \frac{1993!}{12!1981!} \qquad\textbf{(D)}\ \frac{1993!}{13!1980!} \qquad\textbf{(E)}\ \text{None of above} $

2004 India Regional Mathematical Olympiad, 7

Tags: inequalities

Let $x$ and $y$ be positive real numbers such that $y^3 + y \leq x - x^3$. Prove that (A) $y < x < 1$ (B) $x^2 + y^2 < 1$.