2018 AIME I Problems/Problem 7
A right hexagonal prism has height . The bases are regular hexagons with side length . Any of the vertices determine a triangle. Find the number of these triangles that are isosceles (including equilateral triangles).
We can consider two cases: when the three vertices are on one base, and when the vertices are on two bases.
Case 1: vertices are on one base. Then we can call one of the vertices for distinction. Either the triangle can have sides with 6 cases or with 2 cases. This can be repeated on the other base for cases.
Case 2: The vertices span two bases. WLOG call the only vertex on one of the bases . Call the closest vertex on the other base , and label clockwise . We will multiply the following scenarios by , because the top vertex can have positions and the top vertex can be on the other base. We can have , but we are not done! Don't forget that the problem statement implies that the longest diagonal in a base is and the height is , so is also correct! Those are the only three cases, so there are cases for this case.
In total there's cases.
If there are two edges on a single diameter, there would be six diameters. There are four ways to put the third number, and four equilateral triangles. There are ways. Then, if one length was but no side on the diameter, there would be twelve was to put the side, and two ways to put the other point. for four ways to put the third point. Adding the number up, the final answer is
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