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Revision as of 18:13, 13 June 2017
Problem
Given the nine-sided regular polygon , how many distinct equilateral triangles in the plane of the polygon have at least two vertices in the set ?
Solution
Each of the pairs of vertices determines two equilateral triangles, one on each side of the segment. This would give us triangles. However, note that there are three equilateral triangles that have all three vertices among the vertices of the polygon. These are the triangles , , and . We counted each of these three times (once for each side). Hence we overcounted by for each of these triangles for a total of overcounted, and the correct number of equilateral triangles is .
See Also
2001 AMC 12 (Problems • Answer Key • Resources) | |
Preceded by Problem 13 |
Followed by Problem 15 |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | |
All AMC 12 Problems and Solutions |
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