CONO-SUR 2011

Find all triplets of positive integers $(x,y,z)$ such that $x^{2}+y^{2}+z^{2}=2011$ .

The numbers $1$ through $4^{n}$ are written on a board. In each step, Pedro erases two numbers $a$ and $b$ from the board, and writes instead the number $\frac{ab}{\sqrt{2a^2+2b^2}}$ . Pedro repeats this procedure until only one number remains. Prove that this number is less than $\frac{1}{n}$ , no matter what numbers Pedro chose in each step.

Let $ABC$ be an equilateral triangle. Let $P$ be a point inside of it such that the square root of the distance of $P$ to one of the sides is equal to the sum of the square roots of the distances of $P$ to the other two sides. Find the geometric place of $P$ .

A number $\overline{abcd}$ is called balanced if $a+b=c+d$ . Find all balanced numbers with 4 digits that are the sum of two palindrome numbers.

Let $ABC$ be a triangle and $D$ a point in $AC$ . If $\angle{CBD} - \angle{ABD} = 60^{\circ}, \hat{BDC} = 30^{\circ}$ and also $AB \cdot BC = BD^{2}$ , determine the measure of all the angles of triangle $ABC$ .

Let $Q$ be a $(2n+1) \times (2n+1)$ board. Some of its cells are colored black in such a way that every $2 \times 2$ board of $Q$ has at most $2$ black cells. Find the maximum amount of black cells that the board may have.