2019 AMC 12A Problem 8

Attempt Problem 8 of the 2019 AMC 12A 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 12A solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

8.

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
Small hint:
The maximum is (42)=6\binom{4}{2} = 6 when the lines are in general position
Big hint:
Enumerate configurations using parallel classes and concurrences; check which counts from 00 to 66 are achievable
Solution:
The values 00 and 11 occur when all four lines are parallel or all four are concurrent. Three parallel lines crossed by a fourth give 33 points. Three concurrent lines together with a fourth line not through their common point give 4.4. One parallel pair with no three concurrent gives 5,5, and four lines in general position give 6.6.
The value 22 is impossible. Once two lines meet at P,P, a third line not through PP creates at least one new intersection; a fourth distinct line must then create another new point. If every remaining line passes through P,P, there is only one intersection point instead. Thus the achievable values are 0,1,3,4,5,6,0,1,3,4,5,6, whose sum is 19.19.
Thus, the correct answer is D.
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