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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Concepts:counting intersectionscasework

Difficulty rating: 1660

14.

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

Solution:

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. Suppose the only intersection points are XX and YY. If no line passes through both, then the lines through XX must all be parallel to the lines through YY to avoid new intersections, which is impossible because two distinct lines through XX are not parallel. If one line passes through both XX and YY, then any other line through XX and any other line through YY 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 0,1,3,4,5,60,1,3,4,5,6, whose sum is 1919. Thus, D is the correct answer.

Problem 14 in Other Years