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 个不同的点位于两条或更多条直线上。所有可能的 值之和是多少?
For a set of four distinct lines in a plane, there are exactly distinct points that lie on two or more of the lines. What is the sum of all possible values of
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 和 。四条平行线给出 ;四线共点给出 ;三条平行线被第四条截得给出 ;三线共点加一条不过该点的直线给出 ;三条线成三角形,第四条平行于其中一边给出 ;一般位置四条直线给出 。
还需要排除恰好 个交点的情形。取两条不平行的直线,设它们交于 。若第三条直线也经过 ,则任何不经过 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 ,就只有一个交点。若第三条直线不经过 ,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 ,其和为 。正确答案是 D。
The values and are attainable. Four parallel lines give , four concurrent lines give , three parallel lines cut by a fourth give , three concurrent lines plus a fourth not through that point give , three lines forming a triangle plus a fourth parallel to one side give , and four lines in general position give .
It remains to rule out . Choose two nonparallel lines, meeting at . If a third line also passes through , then a fourth line not through intersects at least two of those three concurrent lines at two different new points; otherwise all four lines pass through , giving only one point. If the third line does not pass through , 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 , whose sum is . Thus, D is the correct answer.
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