2025 AIME I 第 10 题

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10.

一个 3×93 \times 9 方格的 2727 个格子用数字 1199 填入,使得每一行包含 99 个不同数字,并且如下例中粗线标出的三个 3×33 \times 3 区块也各包含 99 个不同数字, 就像数独题的前三行一样。

这种方格的不同填法数可写成 paqbrcsdp^a \cdot q^b \cdot r^c \cdot s^d,其中 ppqqrrss 是互不相同的质数,aabbccdd 是正整数。 求 pa+qb+rc+sdp \cdot a + q \cdot b + r \cdot c + s \cdot d

The 2727 cells of a 3×93 \times 9 grid are filled in using the numbers 11 through 99 so that each row contains 99 different numbers, and each of the three 3×33 \times 3 blocks heavily outlined in the example below contains 99 different numbers, as in the first three rows of a Sudoku puzzle.

The number of different ways to fill such a grid can be written as paqbrcsd,p^a \cdot q^b \cdot r^c \cdot s^d, where p,p, q,q, r,r, and ss are distinct prime numbers and a,a, b,b, c,c, dd are positive integers. Find pa+qb+rc+sd.p \cdot a + q \cdot b + r \cdot c + s \cdot d.

答案:81
知识点:有限制的排列乘法原理质因数分解
难度评级:2990
解答:

左侧区块任意填写:有 9!9! 种方式。令 R1,R2,R3R_1, R_2, R_3 为其三行中的三个数字集合。 在中间区块中,第 ii 行必须避开 RiR_i,因为这些数字已经在第 ii 行出现过,并且该区块的三行必须划分 {1,,9}\{1, \ldots, 9\}。设中间区块第一行从 R2R_2 中取 jj 个数字、从 R3R_3 中取 3j3 - j 个数字。平衡三行后,中间行被迫取 R1R_1 中的 3j3 - j 个数字以及 R3R_3 中剩下的全部 jj 个数字,底行也随之确定。数字集合的选择数为 j=03(3j)(33j)2=1+27+27+1=56. \begin{gathered} \sum_{j=0}^{3} \binom{3}{j}\binom{3}{3-j}^2 \\ = 1 + 27 + 27 + 1 \\ = 56. \end{gathered}

右侧区块的各行数字集合随后被强制确定,即第 ii 行取第 ii 行还缺少的数字;中间和右侧区块共六行, 每行内部可按 3!3! 种方式排列。总数为 9!5666=(273457)(237)(2636)=2163105172. \begin{gathered} 9! \cdot 56 \cdot 6^6 \\ = (2^7 \cdot 3^4 \cdot 5 \cdot 7)(2^3 \cdot 7)(2^6 \cdot 3^6) \\ = 2^{16} \cdot 3^{10} \cdot 5^1 \cdot 7^2. \end{gathered}

因此 pa+qb+rc+sdp \cdot a + q \cdot b + r \cdot c + s \cdot d =216+310+51= 2 \cdot 16 + 3 \cdot 10 + 5 \cdot 1 +72=81+ 7 \cdot 2 = 81

Fill the left block arbitrarily: 9!9! ways. Let R1,R2,R3R_1, R_2, R_3 be the sets of three digits in its rows. In the middle block, row ii must avoid RiR_i (those digits already appear in row ii), and the block's three rows must partition {1,,9}.\{1, \ldots, 9\}. Say its top row takes jj digits from R2R_2 and 3j3 - j from R3.R_3. Balancing the three rows then forces the middle row to take 3j3 - j digits from R1R_1 together with all jj remaining digits of R3,R_3, and the bottom row is determined. The number of content choices is j=03(3j)(33j)2=1+27+27+1=56. \begin{gathered} \sum_{j=0}^{3} \binom{3}{j}\binom{3}{3-j}^2 \\ = 1 + 27 + 27 + 1 \\ = 56. \end{gathered}

The right block's row contents are then forced (row ii takes whatever is missing from row ii), and each of the six rows of the middle and right blocks can be ordered internally in 3!3! ways. The total is 9!5666=(273457)(237)(2636)=2163105172. \begin{gathered} 9! \cdot 56 \cdot 6^6 \\ = (2^7 \cdot 3^4 \cdot 5 \cdot 7)(2^3 \cdot 7)(2^6 \cdot 3^6) \\ = 2^{16} \cdot 3^{10} \cdot 5^1 \cdot 7^2. \end{gathered}

Therefore pa+qb+rc+sdp \cdot a + q \cdot b + r \cdot c + s \cdot d =216+310+51= 2 \cdot 16 + 3 \cdot 10 + 5 \cdot 1 +72=81.+ 7 \cdot 2 = 81.

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