2019 AMC 10A 第 14 题

先试着解答 2019 AMC 10A 第 14 题,然后核对你的答案与精心整理的解答,解答来自 LIVE by Po-Shen Loh。你也可以参加完整限时模拟考试、查看全部 2019 AMC 10A 解答,或核对答案

所有题目均经美国数学协会(MAA)官方合法授权使用。

14.

平面中有四条不同的直线,恰好有 NN 个不同的点位于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:1660
小提示:

列出四条直线能达到的交点数。

List attainable intersection counts for four lines

大提示:

再单独排除恰好两个交点的情况。

Then separately rule out exactly two intersection points

解答:

可达到的值有 0,1,3,4,50,1,3,4,566。四条平行线给出 00;四线共点给出 11;三条平行线被第四条截得给出 33;三线共点加一条不过该点的直线给出 44;三条线成三角形,第四条平行于其中一边给出 55;一般位置四条直线给出 (42)=6\binom42=6

还需要排除恰好 22 个交点的情形。取两条不平行的直线,设它们交于 XX。若第三条直线也经过 XX,则任何不经过 XX 的第四条直线,都会与这三条共点直线中的至少两条交于两个不同的新点;否则四条直线全都经过 XX,就只有一个交点。若第三条直线不经过 XX,那么它要只产生一个新交点,就必须与前两条直线之一平行。此时第四条直线若经过已有的某个交点,就一定会与那条平行线交于新的点;若两个已有交点都不经过,它会立刻产生新的交点。因此恰好两个交点是不可能的。

所以可能值为 0,1,3,4,5,60,1,3,4,5,6,其和为 1919。正确答案是 D

The values 0,1,3,4,5,0,1,3,4,5, and 66 are attainable. Four parallel lines give 00, four concurrent lines give 11, three parallel lines cut by a fourth give 33, three concurrent lines plus a fourth not through that point give 44, three lines forming a triangle plus a fourth parallel to one side give 55, and four lines in general position give (42)=6\binom42=6.

It remains to rule out 22. Choose two nonparallel lines, meeting at XX. If a third line also passes through XX, then a fourth line not through XX intersects at least two of those three concurrent lines at two different new points; otherwise all four lines pass through XX, giving only one point. If the third line does not pass through XX, then to create only one new point it must be parallel to one of the first two lines. A fourth distinct line cannot pass through either existing intersection without meeting the parallel line at a new point, and if it passes through neither, it creates a new intersection immediately. Thus exactly two intersection points are impossible.

Thus the possible values are 0,1,3,4,5,60,1,3,4,5,6, whose sum is 1919. Thus, D is the correct answer.

第 13 题#13
完整试卷

其他年份的第 14 题