# 2012 USAJMO Problems真题及答案

+答案解析请参考文末

## Day 1

### Problem 1

Given a triangle $ABC$, let $P$ and $Q$ be points on segments $\overline{AB}$ and $\overline{AC}$, respectively, such that $AP = AQ$. Let $S$ and $R$ be distinct points on segment $\overline{BC}$ such that $S$ lies between $B$ and $R$ $\angle BPS = \angle PRS$, and $\angle CQR = \angle QSR$. Prove that $P$ $Q$ $R$ $S$ are concyclic (in other words, these four points lie on a circle).

### Problem 2

Find all integers $n \ge 3$ such that among any $n$ positive real numbers $a_1$ $a_2$ $\dots$ $a_n$ with $$\max(a_1, a_2, \dots, a_n) \le n \cdot \min(a_1, a_2, \dots, a_n),$$there exist three that are the side lengths of an acute triangle.

### Problem 3

Let $a$ $b$ $c$ be positive real numbers. Prove that $$\frac{a^3 + 3b^3}{5a + b} + \frac{b^3 + 3c^3}{5b + c} + \frac{c^3 + 3a^3}{5c + a} \ge \frac{2}{3} (a^2 + b^2 + c^2).$$

## Day 2

### Problem 4

Let $\alpha$ be an irrational number with $0 < \alpha < 1$, and draw a circle in the plane whose circumference has length 1. Given any integer $n \ge 3$, define a sequence of points $P_1$ $P_2$ $\dots$ $P_n$ as follows. First select any point $P_1$ on the circle, and for $2 \le k \le n$ define $P_k$ as the point on the circle for which the length of arc $P_{k - 1} P_k$ is $\alpha$, when travelling counterclockwise around the circle from $P_{k - 1}$ to $P_k$. Suppose that $P_a$ and $P_b$ are the nearest adjacent points on either side of $P_n$. Prove that $a + b \le n$.

### Problem 5

For distinct positive integers $a$ $b < 2012$, define $f(a,b)$ to be the number of integers $k$ with $1 \le k < 2012$ such that the remainder when $ak$divided by 2012 is greater than that of $bk$ divided by 2012. Let $S$ be the minimum value of $f(a,b)$, where $a$ and $b$ range over all pairs of distinct positive integers less than 2012. Determine $S$.

### Problem 6

Let $P$ be a point in the plane of triangle $ABC$, and $\gamma$ a line passing through $P$. Let $A'$ $B'$ $C'$ be the points where the reflections of lines $PA$ $PB$ $PC$ with respect to $\gamma$ intersect lines $BC$ $AC$ $AB$, respectively. Prove that $A'$ $B'$ $C'$ are collinear.

### 2012USAJMO真题参考答案及详解 ## 报名及辅导请联系  Aaron 李老师 15618605663  微信：linstitute4 