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
答案:D
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 。
恰好 个交点不可能。设仅有交点 和 。若没有直线同时经过两点,则经过 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 的两条不同直线不可能平行。若一条直线同时经过 和 ,则任何另一条经过 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 ,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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