Asian Pacific Mathematics Olympiad · 2016

Problems

Problem 1We say that a triangle $ABC$ is great if the following holds: for any point $D$ on side $BC$ , if $P$ and $Q$ are the feet of the perpendiculars from $D$ to the lines $AB$ and $AC$ respectively, then the reflection of $D$ in the line $PQ$ lies on the circumcircle of triangle $ABC$ . Prove that triangle $ABC$ is great if and only if $\angle A=90^\circ$ and $AB=AC$ .Solutions: 1
Problem 2A positive integer is called fancy if it can be expressed in the form $2^{a_1}+2^{a_2}+\cdots+2^{a_{100}}$ , where $a_1,a_2,\ldots,a_{100}$ are non-negative integers that are not necessarily distinct. Find the smallest positive integer $n$ such that no multiple of $n$ is a fancy number.Solutions: 1
Problem 3Let $AB$ and $AC$ be two distinct rays not lying on the same line, and let $\omega$ be a circle with center $O$ that is tangent to ray $AC$ at $E$ and ray $AB$ at $F$ . Let $R$ be a point on segment $EF$ . The line through $O$ parallel to $EF$ intersects line $AB$ at $P$ . Let $N$ be the intersection of lines $PR$ and $AC$ , and let $M$ be the intersection of line $AB$ and the line through $R$ parallel to $AC$ . Prove that line $MN$ is tangent to $\omega$ .Solutions: 1
Problem 4The country Dreamland consists of $2016$ cities. The airline Starways wants to establish some one-way flights between pairs of cities in such a way that each city has exactly one flight out of it. Find the smallest positive integer $k$ such that no matter how Starways establishes its flights, the cities can always be partitioned into $k$ groups so that from any city it is not possible to reach another city in the same group by using at most $28$ flights.Solutions: 1
Problem 5Find all functions $f:\mathbb{R}^+\to\mathbb{R}^+$ such that $(z+1)f(x+y)=f(xf(z)+y)+f(yf(z)+x)$ for all positive real numbers $x,y,z$ .Solutions: 1