2010 AMC 12A Problem 25

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Concepts:Burnside’s Lemmastars and barscyclic quadrilateral

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 32?32?

560560

564564

568568

14981498

22552255

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 32,32, this means each side is at most 15.15.

First count ordered quadruples (a,b,c,d)(a,b,c,d) of positive integers with a+b+c+d=32a+b+c+d=32 and each entry at most 15.15. Without the upper bound there are (313)=4495;\binom{31}{3}=4495; removing those with some entry at least 1616 subtracts 4(163)=2240,4\binom{16}{3}=2240, leaving 2255.2255.

Rotations of the quadrilateral correspond to cyclic permutations of (a,b,c,d).(a,b,c,d). By Burnside's lemma the number of distinct quadrilaterals is 14(2255+f1+f2+f3),\frac{1}{4}\big(2255+f_1+f_2+f_3\big), where fif_i counts quadruples fixed by rotating ii positions.

A one- or three-step rotation fixes only (8,8,8,8),(8,8,8,8), so f1=f3=1.f_1=f_3=1. A two-step rotation fixes (a,b,a,b)(a,b,a,b) with a+b=16a+b=16 and 1a,b15,1\le a,b\le15, giving f2=15.f_2=15.

Hence the count is 14(2255+1+15+1)=22724=568.\frac{1}{4}(2255+1+15+1)=\frac{2272}{4}=568.

Thus, C is the correct answer.

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