# Blog Archives

## CGMO-2012 (China Girls Math Olympiad 2012) Problem 8

Find the number of integers $k$ in the set $\{0, 1, 2,\cdots, 2012\}$   such that  $\binom{2012}{k}$ is a multiple of $2012$

## CGMO – 2012 ( China Girls Math Olympiad 2012 ) Problem 7

Let  $\{a_n\}$ be a sequence of nondecreasing positive integers such that  $\frac{r}{a_{r}}= k+1$ for some positive integers $k$ and $r$. Prove that there exists a positive integer $s$ such that  $\frac{s}{a_s} = k$

## CGMO 2012 ( China Girls Math Olympiad 2012) Problem 6

There are $n$ cities, $2$ airline companies in a country. Between any two cities, there is exactly one $2$-way flight connecting them which is operated by one of the two companies. A female mathematician plans a travel route, so that it starts and ends at the same city, passes through at least two other cities, and each city in the route is visited once. She finds out that wherever she starts and whatever route she chooses, she must take flights of both companies. Find the maximum value of  $n$

## CGMO 2012 ( China Girls Math Olympiad 2012) Problem 5

As shown in the figure below, the in-circle of  is tangent to sides  and  at  and  respectively, and  is the circumcenter of . Prove that .

## CGMO-2012 (China Girls Math Olympiad 2012) Problem 4

There is a stone at each vertex of a given regular $13$-gon, and the color of each stone is black or white. Prove that we may exchange the position of two stones such that the coloring of these stones are symmetric with respect to some symmetric axis of the $13$-gon

## CGMO-2012 (China Girls Math Olympiad 2012) Problem-3

Find all pairs $(a,b)$ of integers satisfying: there exists an integer  $d \ge 2$ such that $a^n + b^n+1$  is divisible by $d$ for all positive integers $n$

## CGMO 2012 (China Girls Math Olympiad 2012)- Problem 2

Circles  and  are tangent to each other externally at . Points  and  are on , lines  and  are tangent to  at  and , respectively, lines  and  meet at point
Prove that

(1) ;
(2) .

## CGMO 2012 (China Girls Math Olympiad 2012) Problem 1

Let  $a_{1}, a_{2},\ldots, a_{n}$ be non-negative real numbers. Prove that                                                                          $\LARGE \frac{1}{1+a_{1}}+\frac{ a_{1}}{(1+a_{1})(1+a_{2})}+\frac{ a_{1}a_{2}}{(1+a_{1})(1+a_{2})(1+a_{3})}\cdots+\frac{ a_{1}a_{2}\cdots a_{n-1}}{(1+a_{1})(1+a_{2})\cdots (1+a_{n})}\le 1.$