2016 AMC 12A 第 18 题

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18.

对某个正整数 nn,数 110n3110n^3110110 个正整数因数,其中包括 11110n3110n^3 本身。数 81n481n^4 有多少个正整数因数?

For some positive integer n,n, the number 110n3110n^3 has 110110 positive integer divisors, including 11 and the number 110n3.110n^3. How many positive integer divisors does the number 81n481n^4 have?

110110

191191

261261

325325

425425

答案:D
知识点:因数个数质因数分解
难度评级:1910
解答:

110n3110n^3 能被三个不同质数 2,5,11.2,5,11. 整除。若其质因数指数为 r1,r2,,r_1,r_2,\ldots,(r1+1)(r2+1)=110,(r_1+1)(r_2+1)\cdots=110,其中 110=2511.110=2\cdot5\cdot11. 因为已经至少有三个大于 11 的因数,所以恰好只有三个,这说明没有其他质数整除 n.n.

110n3110n^3 中的每个指数都满足 1(mod3),1\pmod3,因此每个因数个数中的因子 ri+1r_i+1 都满足 2(mod3).2\pmod3. 因数 2,5,112,5,11 都具有这种形式,所以指数按某种顺序为 1,4,101,4,10。从 110110 中已有的指数减去 11,再除以 3,3,得到 nn 中的指数按某种顺序为 0,1,30,1,3。因此 n=pq3n=pq^3,其中不同质数 p,qp,q 选自 2,5,11.2,5,11.

所以 n4n^4 中的指数为 4412,12,81=3481=3^4 引入第三个质数,因为 3n.3\nmid n. 因此 81n481n^4 的因数个数为 (4+1)(4+1)(12+1)=5513=325. \begin{gathered} (4+1)(4+1)(12+1)\\ =5\cdot5\cdot13\\ =325. \end{gathered}

因此,正确答案是 D

The number 110n3110n^3 is divisible by the three distinct primes 2,5,11.2,5,11. If its prime exponents are r1,r2,,r_1,r_2,\ldots, then (r1+1)(r2+1)=110,(r_1+1)(r_2+1)\cdots=110, where 110=2511.110=2\cdot5\cdot11. Because there are already at least three factors greater than 1,1, there are exactly three, so no other prime divides n.n.

Each exponent in 110n3110n^3 is 1(mod3),1\pmod3, so each divisor-count factor ri+1r_i+1 is 2(mod3).2\pmod3. The factors 2,5,112,5,11 all have that form, so the exponents are 1,4,101,4,10 in some order. After subtracting the exponent 11 from 110110 and dividing by 3,3, the exponents in nn are 0,1,30,1,3 in some order. Thus n=pq3n=pq^3 for two distinct primes p,qp,q chosen from 2,5,11.2,5,11.

Consequently n4n^4 has exponents 44 and 12,12, while 81=3481=3^4 introduces a third prime because 3n.3\nmid n. Hence the number of divisors of 81n481n^4 is (4+1)(4+1)(12+1)=5513=325. \begin{gathered} (4+1)(4+1)(12+1)\\ =5\cdot5\cdot13\\ =325. \end{gathered}

Thus, the correct answer is D.

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