2023 AMC 12B 第 23 题

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23.

nn 个标准六面骰子时,掷出的数字乘积可能有 936936 种不同的值。求 nn

When nn standard six-sided dice are rolled, the product of the numbers rolled can be any of 936936 possible values. What is n?n?

1111

66

88

1010

99

答案:A
知识点:质因数分解系统列举
难度评级:2270
解答:

2,3,52,3,5 1(0,0,0)1\to(0,0,0)2(1,0,0)2\to(1,0,0)3(0,1,0)3\to(0,1,0)4(2,0,0)4\to(2,0,0)5(0,0,1)5\to(0,0,1)6(1,1,0)6\to(1,1,0)

每个骰子在质数 55 上贡献一个指数向量(面 kk55m=nkm=n-kbb330bm0\le b\le m),而乘积由这些向量之和决定。 aa 22 0,1,,2mb0,1,\ldots,2m-baba\le baa 66 bab-a 33a>ba\gt bbb 66 aba-b 22 44 22a2mba\le 2m-b mm 11 b=0m(2mb+1)=(m+1)(3m+2)2. \begin{gathered} \sum_{b=0}^{m}(2m-b+1)\\ {}=\frac{(m+1)(3m+2)}2. \end{gathered}

k=0,1,,nk=0,1,\ldots,n 计数不同的可达向量和,得到 m=0,1,,nm=0,1,\ldots,n122132=936\dfrac{12^2\cdot13}{2}=936,所以 n=11n=11n=11n=11m=0n(m+1)(3m+2)2=(n+1)2(n+2)2. \begin{gathered} \sum_{m=0}^{n}\frac{(m+1)(3m+2)}2\\ {}=\frac{(n+1)^2(n+2)}2. \end{gathered}

因此,正确答案是 A

Each die contributes an exponent vector in the primes 2,3,52,3,5 (face 1(0,0,0),1\to(0,0,0), 2(1,0,0),2\to(1,0,0), 3(0,1,0),3\to(0,1,0), 4(2,0,0),4\to(2,0,0), 5(0,0,1),5\to(0,0,1), 6(1,1,0)6\to(1,1,0)), and a product is determined by the sum of these vectors.

Fix the exponent of 55 by requiring exactly kk dice to show 5,5, and put m=nk.m=n-k. For an exponent bb of 3,3, where 0bm,0\le b\le m, the possible exponents aa of 22 are precisely 0,1,,2mb.0,1,\ldots,2m-b. If ab,a\le b, use aa faces showing 66 and bab-a showing 3.3. If a>b,a\gt b, use bb faces showing 66 and make the remaining aba-b factors of 22 with faces 44 and, if needed, one face 2.2. The bound a2mba\le 2m-b says this uses at most mm dice; fill unused dice with 11's. Thus the number of exponent pairs is b=0m(2mb+1)=(m+1)(3m+2)2. \begin{gathered} \sum_{b=0}^{m}(2m-b+1)\\ {}=\frac{(m+1)(3m+2)}2. \end{gathered}

Summing over k=0,1,,nk=0,1,\ldots,n, equivalently over m=0,1,,n,m=0,1,\ldots,n, the number of distinct products is m=0n(m+1)(3m+2)2=(n+1)2(n+2)2. \begin{gathered} \sum_{m=0}^{n}\frac{(m+1)(3m+2)}2\\ {}=\frac{(n+1)^2(n+2)}2. \end{gathered} For n=11n=11 this is 122132=936,\dfrac{12^2\cdot13}{2}=936, so n=11.n=11.

Thus, the correct answer is A.

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