2019 AMC 10A 第 14 题

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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
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

恰好 22 个交点不可能。设仅有交点 XX 和 。若没有直线同时经过两点,则经过 XX 的所有直线都必须与经过 的所有直线平行,才能避免产生新交点;但经过 XX 的两条不同直线不可能平行。若一条直线同时经过 XX 和 ,则任何另一条经过 XX 的直线和任何另一条经过 的直线都必须平行;第四条直线仍会产生额外交点。因此这种情形也不成立。

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.

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