2025 AIME II 第 3 题

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3.

四个单位正方形组成一个 2×22 \times 2 方格。构成这些正方形边的 1212 条单位线段分别被涂成红色或蓝色, 并且每个单位正方形都有 22 条红边和 22 条蓝边。下面给出一个例子(红色为实线,蓝色为虚线)。 求这样的涂色方法数。

Four unit squares form a 2×22 \times 2 grid. Each of the 1212 unit line segments forming the sides of the squares is colored either red or blue in such a way that each unit square has 22 red sides and 22 blue sides. One example is shown below (red is solid, blue is dashed). Find the number of such colorings.

答案:82
知识点:有限制的排列分类讨论乘法原理
难度评级:2440
解答:

1212 条线段分成组成中心十字的 44 条内部线段和 88 条边界线段;每个单位正方形恰有两条内边和两条外边。 先给十字涂色。一个正方形如果已有 jj 条红色内边,就还需要 2j2 - j 条红色外边,可用 (22j)\binom{2}{2-j} 种方式选择:当 j=0j = 0j=2j = 2 时有 11 种,当 j=1j = 1 时有 22 种。

按红色十字臂的集合给 24=162^4 = 16 种十字涂色分组。若四臂同色(22 种涂色),每个正方形都有 j=0j = 0j=2j = 2,各贡献 11 种,总共 22 种。若恰有一臂为红或恰有一臂为蓝(88 种涂色),接触这条特殊臂的两个正方形有 j=1j = 1,另外两个没有,各贡献 22=42 \cdot 2 = 4 种,总共 3232 种。若两条相邻臂为红(44 种涂色),四个正方形的相应数值为 j=2,1,1,0j = 2, 1, 1, 0,每种十字涂色各有 44 种补全方式,总共 1616 种。若两条相对臂为红(22 种涂色),四个正方形都有 j=1j = 1,各贡献 24=162^4 = 16 种,总共 3232 种。

涂色方法数为 2+32+16+32=822 + 32 + 16 + 32 = 82

The 1212 segments split into the 44 interior segments forming the central cross and 88 boundary segments, and each unit square has exactly two interior sides (its two cross arms) and two boundary sides. Color the cross first. A square that already has jj red interior sides needs 2j2 - j red boundary sides, which can be chosen in (22j)\binom{2}{2-j} ways: 11 way if j=0j = 0 or j=2,j = 2, and 22 ways if j=1.j = 1.

Group the 24=162^4 = 16 cross colorings by the set of red arms. If all four arms have the same color (22 colorings), every square has j=0j = 0 or j=2,j = 2, contributing 11 each: total 2.2. If exactly one arm is red or exactly one is blue (88 colorings), the two squares touching the odd arm have j=1j = 1 and the others do not, contributing 22=42 \cdot 2 = 4 each: total 32.32. If two adjacent arms are red (44 colorings), the squares have j=2,1,1,0,j = 2, 1, 1, 0, contributing 44 each: total 16.16. If two opposite arms are red (22 colorings), all four squares have j=1,j = 1, contributing 24=162^4 = 16 each: total 32.32.

The number of colorings is 2+32+16+32=82.2 + 32 + 16 + 32 = 82.

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