2002 AIME II 第 8 题

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8.

求最小的正整数 kk,使方程 ⌊2002n⌋=k\left\lfloor \frac{2002}{n} \right\rfloor = k 没有整数解 nn。(记号 ⌊x⌋\lfloor x \rfloor 表示不大于 xx 的最大整数。)

Find the least positive integer kk for which the equation ⌊2002n⌋=k\left\lfloor \frac{2002}{n} \right\rfloor = k has no integer solutions for n.n. (The notation ⌊x⌋\lfloor x \rfloor means the greatest integer less than or equal to x.x.)

答案:49
知识点:取整函数区间内整数计数
难度评级:2560
小提示:

⌊2002n⌋=k\left\lfloor \frac{2002}{n} \right\rfloor = k 有解,当且仅当某个整数 nn 落在 (2002k+1,2002k]\left(\frac{2002}{k+1}, \frac{2002}{k}\right] 中。

⌊2002n⌋=k\left\lfloor \frac{2002}{n} \right\rfloor = k has a solution exactly when some integer nn lies in (2002k+1,2002k]\left(\frac{2002}{k+1}, \frac{2002}{k}\right]

大提示:

该区间长度为 2002k(k+1)\frac{2002}{k(k+1)},所以当 k(k+1)≤2002k(k+1) \le 2002 时,它一定含有整数。然后计算 ⌊2002n⌋\lfloor \frac{2002}{n} \rfloor 在 n=44n = 44、4343、4242、4141、4040 时的值。

That interval has length 2002k(k+1),\frac{2002}{k(k+1)}, so it always contains an integer when k(k+1)≤2002.k(k+1) \le 2002. Then compute ⌊2002n⌋\lfloor \frac{2002}{n} \rfloor for n=44,n = 44, 43,43, 42,42, 41,41, 40.40.

解答:

值 kk 能被取到,当且仅当某个整数 nn 满足 k≤2002n<k+1k \le \frac{2002}{n} \lt k + 1,也就是区间 (2002k+1,2002k]\left(\frac{2002}{k+1}, \frac{2002}{k}\right] 中含有整数。它的长度为 2002k(k+1)\frac{2002}{k(k+1)};当该长度至少为 11,也就是 k(k+1)≤2002k(k+1) \le 2002 时,所有 k≤44k \le 44 都能被取到。

对更大的 kk,直接检查:n=44n = 44、4343、4242、4141、4040 分别给出 ⌊2002n⌋=45\left\lfloor \frac{2002}{n} \right\rfloor = 45、4646、4747、4848、5050。因为 200241≈48.8\frac{2002}{41} \approx 48.8,而 200240>50\frac{2002}{40} \gt 50,所以不可能取到 4949。因此最小的此类 kk 是 4949。

The value kk is attained exactly when some integer nn satisfies k≤2002n<k+1,k \le \frac{2002}{n} \lt k + 1, that is, when the interval (2002k+1,2002k]\left(\frac{2002}{k+1}, \frac{2002}{k}\right] contains an integer. Its length is 2002k(k+1),\frac{2002}{k(k+1)}, which is at least 11 whenever k(k+1)≤2002k(k+1) \le 2002 — so every k≤44k \le 44 is attained.

For larger k,k, check directly: n=44,n = 44, 43,43, 42,42, 41,41, 4040 give ⌊2002n⌋=45,\left\lfloor \frac{2002}{n} \right\rfloor = 45, 46,46, 47,47, 48,48, 50.50. Since 200241≈48.8\frac{2002}{41} \approx 48.8 and 200240>50,\frac{2002}{40} \gt 50, the value 4949 is never attained, so the least such kk is 49.49.

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