CN-SMO 2006

Suppose $a>b>0$ , $f(x)=\dfrac{2(a+b)x+2ab}{4x+a+b}$ . Show that there exists an unique positive number $x$ , such that $f(x)=\left(\dfrac{a^{\frac{1}{3}}+b^{\frac{1}{3}}}{2} \right)^3$ .

In $\triangle ABC$ , $\angle ABC=90^{\circ}$ . Points $D,G$ lie on side $AC$ . Points $E, F$ lie on segment $BD$ , such that $AE \perp BD$ and $GF \perp BD$ . Show that if $BE=EF$ , then $\angle ABG=\angle DFC$ .

There is a standard deck of $52$ cards without jokers. The deck consists of four suits(diamond, club, heart, spade) which include thirteen cards in each. For each suit, all thirteen cards are ranked from " $2$ " to " $A$ " (i.e. $2, 3,\ldots , Q, K, A$ ). A pair of cards is called a "straight flush" if these two cards belong to the same suit and their ranks are adjacent. Additionally, " $A$ " and " $2$ " are considered to be adjacent (i.e. "A" is also considered as " $1$ "). For example, spade $A$ and spade $2$ form a "straight flush"; diamond $10$ and diamond $Q$ are not a "straight flush" pair. Determine how many ways of picking thirteen cards out of the deck such that all ranks are included but no "straight flush" exists in them.

Given any positive integer $n$ , let $a_n$ be the real root of equation $x^3+\dfrac{x}{n}=1$ . Prove that (1) $a_{n+1}>a_n$ ; (2) $\sum_{i=1}^{n}\frac{1}{(i+1)^2a_i} <a_n$ .

In $\triangle ABC$ , $\angle A=60^\circ$ . $\odot I$ is the incircle of $\triangle ABC$ . $\odot I$ is tangent to sides $AB$ , $AC$ at $D$ , $E$ , respectively. Line $DE$ intersects line $BI$ and $CI$ at $F$ , $G$ respectively. Prove that $FG=\frac{BC}{2}$ .

Find the minimum value of real number $m$ , such that inequality $$m(a^3+b^3+c^3) \ge 6(a^2+b^2+c^2)+1$$ holds for all positive real numbers $a,b,c$ where $a+b+c=1$ .

(1) Find the number of positive integer solutions $(m,n,r)$ of the indeterminate equation $mn+nr+mr=2(m+n+r)$ .(2) Given an integer $k (k>1)$ , prove that indeterminate equation $mn+nr+mr=k(m+n+r)$ has at least $3k+1$ positive integer solutions $(m,n,r)$ .

Given a circle with its perimeter equal to $n$ ( $n \in N^*$ ), the least positive integer $P_n$ which satisfies the following condition is called the "number of the partitioned circle": there are $P_n$ points ( $A_1,A_2, \ldots ,A_{P_n}$ ) on the circle; For any integer $m$ ( $1\le m\le n-1$ ), there always exist two points $A_i,A_j$ ( $1\le i,j\le P_n$ ), such that the length of arc $A_iA_j$ is equal to $m$ . Furthermore, all arcs between every two adjacent points $A_i,A_{i+1}$ ( $1\le i\le P_n$ , $A_{p_n+1}=A_1$ ) form a sequence $T_n=(a_1,a_2,,,a_{p_n})$ called the "sequence of the partitioned circle". For example when $n=13$ , the number of the partitioned circle $P_{13}$ =4, the sequence of the partitioned circle $T_{13}=(1,3,2,7)$ or $(1,2,6,4)$ . Determine the values of $P_{21}$ and $P_{31}$ , and find a possible solution of $T_{21}$ and $T_{31}$ respectively.