2007 BMO Problems: Difference between revisions

Revision as of 18:26, 30 April 2007

Problems of the 2007 Balkan Mathematical Olympiad.

Problem 1

Let $\displaystyle ABCD$ be a convex quadrilateral with $\displaystyle AB=BC=CD$ and $\displaystyle AC$ not equal to $\displaystyle BD$ , and let $\displaystyle E$ be the intersection point of its diagonals. Prove that $\displaystyle AE=DE$ if and only if $\angle BAD+\angle ADC = 120^{\circ}$ .

Solution

Problem 2

Find all functions $\displaystyle f : \mathbb{R} \mapsto \mathbb{R}$ such that

$\displaystyle f(f(x) + y) = f(f(x) - y) + 4f(x)y$ .

Solution

Problem 3

Find all positive integers $\displaystyle n$ such that there exists a permutation $\displaystyle \sigma$ on the set $\{ 1, \ldots, n \}$ for which

$\sqrt{\sigma(1) + \sqrt{\sigma(2) + \sqrt{ \cdots + \sqrt{\sigma(n-1) + \sqrt{\sigma(n)}}}}}$

is a rational number.

Solution

Problem 4

For a given positive integer $\displaystyle n >2$ , let $\displaystyle C_{1},C_{2},C_{3}$ be the boundaries of three convex $\displaystyle n$ -gons in the plane such that $C_{1}\cap C_{2}$ , $C_{2}\cap C_{3}$ , $C_{1}\cap C_{3}$ are finite. Find the maximum number of points in the set $C_{1}\cap C_{2}\cap C_{3}$ .