2010 AMC 12A Problem 25
Below is the professionally curated solution for Problem 25 of the 2010 AMC 12A, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2010 AMC 12A solutions, or check the answer key.
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Difficulty rating: 2520
25.
Two quadrilaterals are considered the same if one can be obtained from the other by a rotation and a translation. How many different convex cyclic quadrilaterals are there with integer sides and perimeter equal to
Solution:
A convex cyclic quadrilateral is determined up to rotation and translation by its cyclic sequence of side lengths, and it exists exactly when the largest side is less than the sum of the others. With perimeter this means each side is at most
First count ordered quadruples of positive integers with and each entry at most Without the upper bound there are removing those with some entry at least subtracts leaving
Rotations of the quadrilateral correspond to cyclic permutations of By Burnside's lemma the number of distinct quadrilaterals is where counts quadruples fixed by rotating positions.
A one- or three-step rotation fixes only so A two-step rotation fixes with and giving
Hence the count is
Thus, C is the correct answer.
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