2010 AIME II 第 9 题

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9.

ABCDEFABCDEF 是正六边形。令 GGHHIIJJKKLL 分别为边 ABABBCBCCDCDDEDEEFEFAFAF 的中点。线段 AHAHBIBICJCJDKDKELELFGFG 围成一个较小的正六边形。若较小六边形面积与 ABCDEFABCDEF 面积之比写成最简分数 mn\frac{m}{n},其中 mmnn 是互质正整数,求 m+nm + n

Let ABCDEFABCDEF be a regular hexagon. Let G,G, H,H, I,I, J,J, K,K, and LL be the midpoints of sides AB,AB, BC,BC, CD,CD, DE,DE, EF,EF, and AF,AF, respectively. The segments AH,AH, BI,BI, CJ,CJ, DK,DK, EL,EL, and FGFG bound a smaller regular hexagon. Let the ratio of the area of the smaller hexagon to the area of ABCDEFABCDEF be expressed as a fraction mn,\frac{m}{n}, where mm and nn are relatively prime positive integers. Find m+n.m + n.

答案:11
知识点:正多边形坐标几何面积比对称性
难度评级:2430
小提示:

将中心放在原点;由对称性,较小六边形是正的且同心,所以只需求中心到其中一个顶点的距离。

Place the center at the origin; by symmetry the smaller hexagon is regular and concentric, so it suffices to find the distance from the center to one of its vertices

大提示:

小六边形的一个顶点是直线 AHAHFGFG 的交点;当外接圆半径为 11 时,该点为 (57,37)\left(\frac{5}{7}, \frac{\sqrt{3}}{7}\right)

One vertex of the small hexagon is the intersection of lines AHAH and FG;FG; with circumradius 11 it is (57,37)\left(\frac{5}{7}, \frac{\sqrt{3}}{7}\right)

解答:

将六边形中心放在原点,外接圆半径为 11A=(1,0)A = (1, 0)B=(12,32)B = \left(\frac{1}{2}, \frac{\sqrt{3}}{2}\right)C=(12,32)C = \left(-\frac{1}{2}, \frac{\sqrt{3}}{2}\right),且 F=(12,32)F = \left(\frac{1}{2}, -\frac{\sqrt{3}}{2}\right)。于是 H=(0,32)H = \left(0, \frac{\sqrt{3}}{2}\right)G=(34,34)G = \left(\frac{3}{4}, \frac{\sqrt{3}}{4}\right)。旋转 6060^\circ 会置换这六条线段,所以较小六边形是正六边形且与原六边形同心,面积比等于中心到顶点距离之比的平方。

一个顶点是 AHAHFGFG 的交点。直线 AHAHx+23y=1x + \frac{2}{\sqrt{3}}\,y = 1,直线 FGFGy=33x23y = 3\sqrt{3}\,x - 2\sqrt{3}。代入得 x+6x4=1x + 6x - 4 = 1,所以 x=57x = \frac{5}{7}y=37y = \frac{\sqrt{3}}{7}

该顶点到中心的距离平方为 2549+349=47\frac{25}{49} + \frac{3}{49} = \frac{4}{7},而 AA 到中心距离为 11。面积比为 47\frac{4}{7},所以 m+n=4+7=11m + n = 4 + 7 = 11

Center the hexagon at the origin with circumradius 1:1: A=(1,0),A = (1, 0), B=(12,32),B = \left(\frac{1}{2}, \frac{\sqrt{3}}{2}\right), C=(12,32),C = \left(-\frac{1}{2}, \frac{\sqrt{3}}{2}\right), and F=(12,32).F = \left(\frac{1}{2}, -\frac{\sqrt{3}}{2}\right). Then H=(0,32)H = \left(0, \frac{\sqrt{3}}{2}\right) and G=(34,34).G = \left(\frac{3}{4}, \frac{\sqrt{3}}{4}\right). Rotation by 6060^\circ permutes the six segments, so the smaller hexagon is regular and concentric, and the area ratio is the square of the ratio of distances from the center to a vertex.

One vertex is the intersection of AHAH and FG.FG. Line AHAH is x+23y=1,x + \frac{2}{\sqrt{3}}\,y = 1, and line FGFG is y=33x23.y = 3\sqrt{3}\,x - 2\sqrt{3}. Substituting gives x+6x4=1,x + 6x - 4 = 1, so x=57x = \frac{5}{7} and y=37.y = \frac{\sqrt{3}}{7}.

That vertex has squared distance 2549+349=47\frac{25}{49} + \frac{3}{49} = \frac{4}{7} from the center, while AA is at distance 1.1. The ratio of areas is 47,\frac{4}{7}, and m+n=4+7=11.m + n = 4 + 7 = 11.

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