2019 AMC 12A 第 8 题

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所有题目均经美国数学协会(MAA)官方合法授权使用。

8.

对于平面上的四条互不相同的直线,恰有 NN 个不同的点同时位于其中两条或更多条直线上。所有可能的 NN 值之和是多少?

For a set of four distinct lines in a plane, there are exactly NN distinct points that lie on two or more of the lines. What is the sum of all possible values of N?N?

1414

1616

1818

1919

2121

答案:D
知识点:交点计数分类讨论
难度评级:1380
解答:

当四条直线全部平行或全部共点时,交点数分别为 0011。三条平行线被第四条截时有 33 个交点。三条共点直线加上一条不经过公共点的直线时有 4.4. 个交点。一对平行且没有三线共点时有 5,5, 个交点;四条直线处于一般位置时有 6.6. 个交点。

数值 22 不可能。两条直线在 P,P, 点相交后,第三条若不经过 PP,就至少产生一个新交点;第四条不同的直线还必须产生另一个新交点。如果其余直线都经过 P,P,则只有一个交点。因此可实现的值为 0,1,3,4,5,6,0,1,3,4,5,6,它们的和为 19.19.

因此,正确答案是 D

The values 00 and 11 occur when all four lines are parallel or all four are concurrent. Three parallel lines crossed by a fourth give 33 points. Three concurrent lines together with a fourth line not through their common point give 4.4. One parallel pair with no three concurrent gives 5,5, and four lines in general position give 6.6.

The value 22 is impossible. Once two lines meet at P,P, a third line not through PP creates at least one new intersection; a fourth distinct line must then create another new point. If every remaining line passes through P,P, there is only one intersection point instead. Thus the achievable values are 0,1,3,4,5,6,0,1,3,4,5,6, whose sum is 19.19.

Thus, the correct answer is D.

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