2016 AIME I 第 12 题

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12.

求最小的正整数 mm,使得 m2−m+11m^2 - m + 11 是至少四个质数的乘积,这些质数不一定互不相同。

Find the least positive integer mm such that m2−m+11m^2 - m + 11 is a product of at least four not necessarily distinct primes.

答案:132
知识点:质因数分解模运算极限情形界定
难度评级:3160
小提示:

分别对 22、33、55 和 77 取模检查 m2−m+11m^2 - m + 11:它永远不能被这些数整除。

Check m2−m+11m^2 - m + 11 modulo 2,2, 3,3, 5,5, and 7:7: it is never divisible by any of them

大提示:

所以每个质因数至少为 1111。测试候选值 11411^4 和 113⋅1311^3 \cdot 13,并使用 m≡0m \equiv 0 或 1(mod11)1 \pmod{11}。

So every prime factor is at least 11.11. Test the candidates 11411^4 and 113⋅13,11^3 \cdot 13, using m≡0m \equiv 0 or 1(mod11).1 \pmod{11}.

解答:

令 e(m)=m2−m+11e(m) = m^2 - m + 11。因为 m2−mm^2 - m 总是偶数,e(m)e(m) 为奇数。检查所有余数类可知,m2−m+11m^2 - m + 11 模 33、55 或 77 均不为 00,所以 e(m)e(m) 的每个质因数至少为 1111。四个这样的质数的乘积至少为 114=1464111^4 = 14641,两个最小候选值是 11411^4 和 113⋅13=1730311^3 \cdot 13 = 17303。

若 e(m)=14641e(m) = 14641,方程 m2−m−14630=0m^2 - m - 14630 = 0 的判别式为 5852158521,它严格介于 2412=58081241^2 = 58081 和 2422=58564242^2 = 58564 之间,所以没有整数解。对于 e(m)=17303e(m) = 17303:因为 e(m)=m(m−1)+11e(m) = m(m - 1) + 11 必须被 1111 整除,所以 m=11km = 11k 或 m=11k+1m = 11k + 1。尝试 m=11km = 11k,得到 11k2−k+1=157311k^2 - k + 1 = 1573,也就是 k(11k−1)=1572k(11k - 1) = 1572,而 k=12k = 12 满足:12⋅131=157212 \cdot 131 = 1572。

因为 ee 在 m≥1m \ge 1 时递增,所有更小的 mm 都有 e(m)<17303e(m) \lt 17303,而低于它的唯一四质数乘积 11411^4 不可达到。因此最小的 mm 是 11⋅12=13211 \cdot 12 = 132,此时 e(132)=17303=113⋅13e(132) = 17303 = 11^3 \cdot 13。

Let e(m)=m2−m+11.e(m) = m^2 - m + 11. Since m2−mm^2 - m is always even, e(m)e(m) is odd. Checking all residues shows m2−m+11m^2 - m + 11 is never 00 modulo 3,3, 5,5, or 77 either, so every prime factor of e(m)e(m) is at least 11.11. A product of four such primes is at least 114=14641,11^4 = 14641, and the two smallest candidates are 11411^4 and 113⋅13=17303.11^3 \cdot 13 = 17303.

For e(m)=14641,e(m) = 14641, the discriminant of m2−m−14630=0m^2 - m - 14630 = 0 is 58521,58521, which lies strictly between 2412=58081241^2 = 58081 and 2422=58564,242^2 = 58564, so there is no integer solution. For e(m)=17303:e(m) = 17303: since e(m)=m(m−1)+11e(m) = m(m - 1) + 11 must be divisible by 11,11, either m=11km = 11k or m=11k+1.m = 11k + 1. Trying m=11km = 11k gives 11k2−k+1=1573,11k^2 - k + 1 = 1573, that is k(11k−1)=1572,k(11k - 1) = 1572, which k=12k = 12 satisfies: 12⋅131=1572.12 \cdot 131 = 1572.

Since ee is increasing for m≥1,m \ge 1, every smaller mm has e(m)<17303,e(m) \lt 17303, and the only four-prime value below that, 114,11^4, is unattainable. Hence the least mm is 11⋅12=132,11 \cdot 12 = 132, where e(132)=17303=113⋅13.e(132) = 17303 = 11^3 \cdot 13.

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