# 2010-2011 Mock USAJMO Problems/Solutions

## Problems

### Problem 1

Given two fixed, distinct points $B$ and $C$ on plane $\mathcal{P}$, find the locus of all points $A$ belonging to $\mathcal{P}$ such that the quadrilateral formed by point $A$, the midpoint of $AB$, the centroid of $\triangle ABC$, and the midpoint of $AC$ (in that order) can be inscribed in a circle.

### Problem 2

Let $x, y, z$ be positive real numbers such that $x+y+z = 1$. Prove that $$\frac{3x+1}{y+z}+\frac{3y+1}{z+x}+\frac{3z+1}{x+y}\ge\frac{4}{2x+y+z}+\frac{4}{x+2y+z}+\frac{4}{x+y+2z},$$ with equality if and only if $x=y=z$.

### Problem 3

At a certain (lame) conference with $n$ people, each group of three people wishes to meet exactly once. On each of the $d$ days of the conference, every person participates in at most one meeting with two other people, with no limit on the number of groups that can meet. Determine $\min d$ for

a) $n=6$

b) $n=8$

### Problem 4

The sequence $\{a_i\}_{i\ge0}$ satisfies $a_0=1$ and $a_n=\sum_{i=0}^{n-1}(n-i)a_i$ for $n\ge1$. Prove that for all nonnegative integers $m$, we have $$\sum_{k=0}^{m}\frac{a_k}{4^k} < \frac{9}{5}.$$

### Problem 5

Find all solutions to $$5\cdot4^{3m+1}-4^{2m+1}-1=15n^{2m},$$ where $m$ and $n$ are positive integers.

### Problem 6

In convex, tangential quadrilateral $ABCD$, let the incircle touch the four sides $AB, BC, CD, DA$ at points $E, F, G, H$, respectively. Prove that $$\frac{[EFGH]}{[ABCD]} \le \frac{1}{2}.$$ Note: A tangential quadrilateral has concurrent angle bisectors, and $[X]$ denotes the area of figure $X$.