2019 AMC 10A Problem 14
Below is the professionally curated solution for Problem 14 of the 2019 AMC 10A, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2019 AMC 10A solutions, or check the answer key.
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Difficulty rating: 1660
14.
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
Solution:
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 . Suppose the only intersection points are and . If no line passes through both, then the lines through must all be parallel to the lines through to avoid new intersections, which is impossible because two distinct lines through are not parallel. If one line passes through both and , then any other line through and any other line through must be parallel; the fourth line still creates an additional intersection unless it is parallel to both, which cannot cover both points.
Thus the possible values are , whose sum is . Thus, D is the correct answer.
Problem 14 in Other Years
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