2016 AMC 12B 第 24 题

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24.

恰有 77,00077{,}000 个有序四元组 (a,b,c,d)(a,b,c,d) 满足 gcd(a,b,c,d)=77\gcd(a,b,c,d)=77lcm(a,b,c,d)=n\text{lcm}(a,b,c,d)=nnn 的最小可能值是多少?

There are exactly 77,00077{,}000 ordered quadruples (a,b,c,d)(a,b,c,d) such that gcd(a,b,c,d)=77\gcd(a,b,c,d)=77 and lcm(a,b,c,d)=n.\text{lcm}(a,b,c,d)=n. What is the smallest possible value of n?n?

13,86013{,}860

20,79020{,}790

21,56021{,}560

27,72027{,}720

41,58041{,}580

答案:D
知识点:最大公约数最小公倍数质因数分解
难度评级:2550
解答:

将每个数写成 7777 乘以一个约化值,我们需要 gcd=1\gcd=1lcm=m=n/77.\text{lcm}=m=n/77. 对每个整除 mm 且最大指数为 M,M, 的质数 pp,有效指数四元组的个数为 (M+1)42M4(M+1)^4-2M^4 +(M1)4+(M-1)^4 =2(6M2+1).=2(6M^2+1). 所有质数的贡献乘积必须等于 77,000=2353711.77{,}000=2^3\cdot5^3\cdot7\cdot11.M=1,2,3,M=1,2,3, 时,2(6M2+1)2(6M^2+1) 分别等于 14,14, 50,50,110110,并且 1450110=77,000,14\cdot50\cdot110=77{,}000,所以指数 1,2,31,2,3 给出一个候选分解。

每个质数恰好贡献一个因数 2,2,所以 m.m. 恰好有三个质因数。它们的奇数因子 6M2+16M^2+1 必须整除 77,000/23=9625.77{,}000/2^3=9625. 检查这些因数得到 M=1,2,3,8,M=1,2,3,8,相应的奇数因子为 7,25,55,385.7,25,55,385. 若取 M=8M=8,其余两个奇数因子的乘积只能为 2525,但每个至少为 7,7,不可能。因此最大指数恰为 1,2,3.1,2,3. 为使 m=n/77,m=n/77, 最小,把最大指数分配给最小质数:m=23325=360,m=2^3\cdot3^2\cdot5=360,所以 n=77360=27,720.n=77\cdot360=27{,}720.

因此,正确答案是 D

Writing each entry as 7777 times a reduced value, we need gcd=1\gcd=1 and lcm=m=n/77.\text{lcm}=m=n/77. For each prime pp dividing mm with maximum exponent M,M, the number of valid exponent quadruples is (M+1)42M4(M+1)^4-2M^4 +(M1)4+(M-1)^4 =2(6M2+1).=2(6M^2+1). The total over all primes must equal 77,000=2353711.77{,}000=2^3\cdot5^3\cdot7\cdot11. Since 2(6M2+1)2(6M^2+1) equals 14,14, 50,50, and 110110 for M=1,2,3,M=1,2,3, and 1450110=77,000,14\cdot50\cdot110=77{,}000, the exponents 1,2,31,2,3 give one candidate factorization.

Every prime contributes exactly one factor of 2,2, so exactly three primes divide m.m. Their odd factors 6M2+16M^2+1 must divide 77,000/23=9625.77{,}000/2^3=9625. Checking the divisors gives M=1,2,3,8,M=1,2,3,8, with odd factors 7,25,55,385.7,25,55,385. The choice M=8M=8 leaves only 2525 for the product of the other two odd factors, but each is at least 7,7, so this is impossible. Therefore the maximum exponents are exactly 1,2,3.1,2,3. To minimize m=n/77,m=n/77, assign the largest exponent to the smallest prime: m=23325=360,m=2^3\cdot3^2\cdot5=360, so n=77360=27,720.n=77\cdot360=27{,}720.

Thus, the correct answer is D.

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