2019 AMC 10A Problem 14

Attempt Problem 14 of the 2019 AMC 10A below, then check your answer against the professionally curated solution 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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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

Answer: D
Concepts:counting intersectionscasework
Difficulty rating: 1660
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. 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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