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 distinct points that lie on two or more of the lines. What is the sum of all possible values of
Answer: D
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 . 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.
Problem 14 in Other Years
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