2010 AMC 12A 第 25 题
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25.
如果一个四边形可以通过旋转和平移得到另一个四边形,则认为这两个四边形相同。边长为整数且周长等于 的不同凸圆内接四边形有多少个?
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
答案:C
解答:
一个凸圆内接四边形由其按圆周顺序排列的边长唯一确定,且存在的条件是最大边长小于其他三边之和。周长为 时,每条边至多为 。
先数正整数有序四元组 ,满足 且每项至多为 。若没有上界限制,有 个;去掉某一项至少为 的情况,需减去 ,剩下 个。
四边形的旋转对应于 的循环置换。由 Burnside 引理,不同四边形的数量为 其中 表示旋转 个位置后不变的四元组数。
旋转一步或三步只固定 ,所以 。旋转两步固定形如 且 、 的四元组,所以 。
因此数量为
所以正确答案是 C。
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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