1952 AMC 12 Problem 31

Attempt Problem 31 of the 1952 AMC 12 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 1952 AMC 12 solutions, or check the answer key.

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31.

Given 1212 points in a plane no three of which are collinear, the number of lines they determine is:

2424

5454

120120

6666

None of these

Answer: D
Concepts:combinationsbasic counting
Difficulty rating: 1150
Small Hint:

Each line is determined by choosing two of the points

Big Hint:

The condition that no three are collinear ensures that different pairs determine different lines

Solution:

Every pair of points determines one line, and no line is counted by more than one pair because no three points are collinear. Therefore the number of lines is (122)=12112=66. \binom{12}{2}=\frac{12\cdot11}{2}=66.

Thus, the correct answer is D.

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Problem 31 in Other Years

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