2012 AMC 12A 第 23 题

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23.

SS 是一个正方形,它的一条对角线端点为 (0.1,0.7)(0.1, 0.7)(0.1,0.7)(-0.1, -0.7)。从所有实数 xxyy 满足 0x20120 \le x \le 20120y20120 \le y \le 2012 的有序对中,均匀随机选取点 v=(x,y)v = (x, y)。设 T(v)T(v)SS 的平移副本,并以 vv 为中心。由 T(v)T(v) 确定的正方形区域内部恰好包含两个整数坐标点的概率是多少?

Let SS be the square one of whose diagonals has endpoints (0.1,0.7)(0.1, 0.7) and (0.1,0.7).(-0.1, -0.7). A point v=(x,y)v = (x, y) is chosen uniformly at random over all pairs of real numbers xx and yy such that 0x20120 \le x \le 2012 and 0y2012.0 \le y \le 2012. Let T(v)T(v) be a translated copy of SS centered at v.v. What is the probability that the square region determined by T(v)T(v) contains exactly two points with integer coordinates in its interior?

0.1250.125

0.140.14

0.160.16

0.250.25

0.320.32

答案:C
知识点:几何概率格点面积
难度评级:2340
解答:

(0.1,0.7)(0.1, 0.7)(0.1,0.7)(-0.1, -0.7) 的对角线长度为 0.22+1.42=2,\sqrt{0.2^2 + 1.4^2} = \sqrt2,所以 SS 是面积为 1.1. 的正方形。平移图形 T(v)T(v) 包含一个格点,当且仅当 vv 位于以该格点为中心的 SS 副本内部。

内部恰好包含两个格点,要求 vv 位于以两个相邻格点为中心的副本的重叠区域。由周期性,答案等于一个单位格内所有这类重叠区域的总面积。

考虑以 (0,0)(0,0)(1,0).(1,0). 为中心的两个副本。它们的重叠区域是一个长方形。将所给半对角线旋转四分之一圈,可得到相关顶点 (0.7,0.1)(0.7,-0.1)(0.3,0.1).(0.3,0.1). 沿两组平行边找到交点,可得边长为 0.40.40.2,0.2,所以重叠面积是 0.08.0.08. 对角位置的副本不重叠,而水平和竖直相邻的情形贡献相同。计入单位格边界后,概率为 20.08=0.16.2\cdot0.08=0.16.

因此,正确答案是 C

The diagonal from (0.1,0.7)(0.1, 0.7) to (0.1,0.7)(-0.1, -0.7) has length 0.22+1.42=2,\sqrt{0.2^2 + 1.4^2} = \sqrt2, so SS is a square of area 1.1. The translate T(v)T(v) contains a lattice point exactly when vv lies inside the copy of SS centered at that point.

Containing exactly two interior lattice points requires vv to lie in the overlap of two copies centered at adjacent lattice points. By periodicity the answer is the total such overlap area within one unit cell.

Consider copies centered at (0,0)(0,0) and (1,0).(1,0). Their overlap is a rectangle. A quarter-turn of the listed half-diagonal gives the relevant vertices (0.7,0.1)(0.7,-0.1) and (0.3,0.1).(0.3,0.1). Following the two pairs of parallel sides through their intersections gives side lengths 0.40.4 and 0.2,0.2, so the overlap area is 0.08.0.08. Diagonally centered copies do not overlap, and horizontal and vertical adjacencies contribute equally. Accounting for the cell boundaries gives probability 20.08=0.16.2\cdot0.08=0.16.

Thus, the correct answer is C.

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