2017 AIME II Problems/Problem 13
Problem
For each integer , let
be the number of
-element subsets of the vertices of the regular
-gon that are the vertices of an isosceles triangle (including equilateral triangles). Find the sum of all values of
such that
.
Solution
Considering , we have the following formulas:
Even and a multiple of 3:
Even and not a multiple of 3:
Odd and a multiple of 3:
Odd and not a multiple of 3:
Considering the six possibilities and solving, we find that the only valid solutions are
, from which the answer is
.
See Also
2017 AIME II (Problems • Answer Key • Resources) | ||
Preceded by Problem 12 |
Followed by Problem 14 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
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