2011 AMC 12A 第 22 题

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22.

RR 为一个正方形区域,n4n \ge 4 为整数。若从 RR 内部一点 XX 发出 nn 条射线,可以把 RR 分成 nn 个面积相等的三角形,则称 XXnn-射线分割点。有多少个点是 100100-射线分割点但不是 6060-射线分割点?

Let RR be a square region and n4n \ge 4 an integer. A point XX in the interior of RR is called nn-ray partitional if there are nn rays emanating from XX that divide RR into nn triangles of equal area. How many points are 100100-ray partitional but not 6060-ray partitional?

15001500

15601560

23202320

24802480

25002500

答案:C
知识点:面积格点补集计数
难度评级:2460
解答:

将正方形缩放为 [0,1]2[0,1]^2,并令 X=(x,y).X=(x,y). 射线必须包括通向四个顶点的射线。每个小三角形的面积都是 1/n.1/n. 以底边各段为底的三角形面积之和为 y/2,y/2, 所以这样的三角形有 ny/2.ny/2. 个。同理,沿上、左、右三边的个数分别是 n(1y)/2,n(1-y)/2, nx/2,nx/2,n(1x)/2.n(1-x)/2.

这四个数都必须是正整数。因此 nn 是偶数,并且 X=(2in,2jn),1i,jn21. \begin{gathered} X=\left(\dfrac{2i}{n},\dfrac{2j}{n}\right), \\ 1\le i,j\le\dfrac n2-1. \end{gathered} 反过来,按上述数量把每条边等分,并将分点连接到 XX,就会得到 nn 个等面积三角形。因此这些恰好是分割点。

n=100,n=100, 时,这些点为 (i/50,j/50)(i/50,j/50),其中 1i,j49,1\le i,j\le49,492=2401.49^2=2401. 个。这样的点同时也是 6060 射线分割点,当且仅当对某些整数 c,d,c,d,i/50=c/30i/50=c/30j/50=d/30.j/50=d/30. 因此 iijj 都必须是 5.5. 的倍数。每个坐标有 99 种选择,所以重合的点有 92=819^2=81 个。

所以所求数量为 240181=2320.2401 - 81 = 2320.

因此,正确答案是 C

Scale the square to [0,1]2[0,1]^2 and write X=(x,y).X=(x,y). The rays must include those through the four vertices. Every small triangle has area 1/n.1/n. The triangles whose bases partition the bottom side together have area y/2,y/2, so their number is ny/2.ny/2. Similarly, the numbers along the top, left, and right sides are n(1y)/2,n(1-y)/2, nx/2,nx/2, and n(1x)/2.n(1-x)/2.

These four numbers must be positive integers. Hence nn is even and X=(2in,2jn),1i,jn21. \begin{gathered} X=\left(\dfrac{2i}{n},\dfrac{2j}{n}\right), \\ 1\le i,j\le\dfrac n2-1. \end{gathered} Conversely, partitioning each side into the indicated number of equal segments and joining the division points to XX produces nn equal-area triangles. Thus these are exactly the partitional points.

For n=100,n=100, the points are (i/50,j/50)(i/50,j/50) with 1i,j49,1\le i,j\le49, giving 492=2401.49^2=2401. Such a point is also 6060-ray partitional exactly when i/50=c/30i/50=c/30 and j/50=d/30j/50=d/30 for integers c,d.c,d. Thus ii and jj must both be multiples of 5.5. There are 99 choices for each, so the overlap has 92=819^2=81 points.

So the count is 240181=2320.2401 - 81 = 2320.

Thus, the correct answer is C.

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