2025 AMC 12A 第 18 题

先试着解答 2025 AMC 12A 第 18 题,然后核对你的答案与精心整理的解答,解答来自 LIVE by Po-Shen Loh。你也可以参加完整限时模拟考试、查看全部 2025 AMC 12A 解答,或核对答案

所有题目均经美国数学协会(MAA)官方合法授权使用。

18.

有多少个有序三元组 (x,y,z)(x, y, z),其中三个分量是互不相同且不超过 88 的非负整数,并满足 xy>zxy \gt zzx>yzx \gt yyz>xyz \gt x

How many ordered triples (x,y,z)(x, y, z) of distinct nonnegative integers less than or equal to 88 satisfy xy>z,xy \gt z, zx>y,zx \gt y, and yz>x?yz \gt x?

3636

8484

186186

336336

486486

答案:C
知识点:不等式排列对称性
难度评级:2000
解答:

如果某个变量为 00,例如 z=0z = 0,那么 zx=0>yzx = 0 \gt y 不可能成立。因此 x,y,z{1,,8}x, y, z \in \{1, \ldots, 8\} 是互不相同的正整数。

条件是对称的。对互异的数值 a<b<ca \lt b \lt cac>bac \gt bbc>abc \gt a 自动成立,所以唯一真正的限制是 ab>cab \gt c。只要它成立,所有 66 种排列都可行。

数一数 aaa<b<c8a\lt b\lt c\le8-元素子集 ab>cab\gt c 中满足 a,ba,b 的共有 3131 个。乘以 66 种顺序,得到 ccb+1b+1 min(8,ab1)\min(8,ab-1)631=1866\cdot31=186a123456count01110631. \begin{array}{c|rrrrrr} a&1&2&3&4&5&6\\ \hline \text{count}&0&11&10&6&3&1. \end{array}

因此,正确答案是 C

If any variable is 0,0, say z=0,z = 0, then zx=0>yzx = 0 \gt y is impossible. So x,y,z{1,,8}x, y, z \in \{1, \ldots, 8\} are distinct positive integers.

The conditions are symmetric. For distinct values a<b<c,a \lt b \lt c, we have ac>bac \gt b and bc>abc \gt a automatically, so the only real constraint is ab>c.ab \gt c. When it holds, all 66 orderings work.

For each possible smallest value a,a, the numbers of pairs a<b<c8a\lt b\lt c\le8 satisfying ab>cab\gt c are a123456count01110631. \begin{array}{c|rrrrrr} a&1&2&3&4&5&6\\ \hline \text{count}&0&11&10&6&3&1. \end{array} For fixed a,b,a,b, this count comes from choosing cc between b+1b+1 and min(8,ab1).\min(8,ab-1). Thus there are 3131 unordered triples, and all 66 orderings of each work. The answer is 631=186.6\cdot31=186.

Thus, the correct answer is C.

← 第 17 题#17
完整试卷

其他年份的第 18 题