2017 AIME I 第 13 题

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13.

对每个 m2m \ge 2,令 Q(m)Q(m) 为具有以下性质的最小正整数:对每个 nQ(m)n \ge Q(m),在范围 n<k3mnn \lt k^3 \le m \cdot n 内总存在一个完全立方数 k3k^3。求 m=22017Q(m)\sum_{m=2}^{2017} Q(m) 除以 10001000 的余数。

For every m2,m \ge 2, let Q(m)Q(m) be the least positive integer with the following property: For every nQ(m),n \ge Q(m), there is always a perfect cube k3k^3 in the range n<k3mn.n \lt k^3 \le m \cdot n. Find the remainder when m=22017Q(m)\sum_{m=2}^{2017} Q(m) is divided by 1000.1000.

答案:59
知识点:完全幂极限情形界定分类讨论
难度评级:3160
解答:

如果 k3n<(k+1)3k^3 \le n \lt (k+1)^3,那么只要 (k+1)3mk3(k+1)^3 \le m k^3,也就是 (1+1k)3m\left(1 + \frac{1}{k}\right)^3 \le m,区间 (n,mn](n, mn] 就含有立方数 (k+1)3(k+1)^3。因为对所有 k1k \ge 1 都有 (1+1k)38\left(1 + \frac{1}{k}\right)^3 \le 8,所以每个 m8m \ge 8 都有 Q(m)=1Q(m) = 1

4m74 \le m \le 7n=1n = 1 不满足,因为 (1,m](1, m] 中没有立方数;但对 n2n \ge 2,区间总满足条件:84n8 \le 4n 覆盖 2n72 \le n \le 7,且 (1+1k)3278<4\left(1 + \frac{1}{k}\right)^3 \le \frac{27}{8} \lt 4 覆盖 k2k \ge 2。 所以 Q(4)=Q(5)Q(4) = Q(5) =Q(6)=Q(7)=2= Q(6) = Q(7) = 2。对 m=3m = 3n=8n = 8 不满足 ((8,24](8, 24] 中没有立方数),而 273n27 \le 3n 覆盖 9n269 \le n \le 26,且 (43)3<3\left(\frac{4}{3}\right)^3 \lt 3 覆盖 k3k \ge 3,所以 Q(3)=9Q(3) = 9。对 m=2m = 2n=31n = 31 不满足((31,62](31, 62] 中没有立方数),而 642n64 \le 2n 覆盖 32n6332 \le n \le 63,且 (54)3<2\left(\frac{5}{4}\right)^3 \lt 2 覆盖 k4k \ge 4,所以 Q(2)=32Q(2) = 32

因此 m=22017Q(m)=32+9+42+20101=2059, \begin{aligned} &\sum_{m=2}^{2017} Q(m) \\ &\quad = 32 + 9 + 4 \cdot 2 + 2010 \cdot 1 \\ &\quad = 2059, \end{aligned} 余数为 5959

If k3n<(k+1)3,k^3 \le n \lt (k+1)^3, then the interval (n,mn](n, mn] contains the cube (k+1)3(k+1)^3 as long as (k+1)3mk3,(k+1)^3 \le m k^3, i.e. (1+1k)3m.\left(1 + \frac{1}{k}\right)^3 \le m. Since (1+1k)38\left(1 + \frac{1}{k}\right)^3 \le 8 for all k1,k \ge 1, every m8m \ge 8 has Q(m)=1.Q(m) = 1.

For 4m7:4 \le m \le 7: n=1n = 1 fails since (1,m](1, m] contains no cube, but for n2n \ge 2 the interval works: 84n8 \le 4n covers 2n7,2 \le n \le 7, and (1+1k)3278<4\left(1 + \frac{1}{k}\right)^3 \le \frac{27}{8} \lt 4 covers k2.k \ge 2. So Q(4)=Q(5)Q(4) = Q(5) =Q(6)=Q(7)=2.= Q(6) = Q(7) = 2. For m=3:m = 3: n=8n = 8 fails (no cube in (8,24](8, 24]), while 273n27 \le 3n covers 9n269 \le n \le 26 and (43)3<3\left(\frac{4}{3}\right)^3 \lt 3 covers k3,k \ge 3, so Q(3)=9.Q(3) = 9. For m=2:m = 2: n=31n = 31 fails (no cube in (31,62](31, 62]), while 642n64 \le 2n covers 32n6332 \le n \le 63 and (54)3<2\left(\frac{5}{4}\right)^3 \lt 2 covers k4,k \ge 4, so Q(2)=32.Q(2) = 32.

Therefore m=22017Q(m)=32+9+42+20101=2059, \begin{aligned} &\sum_{m=2}^{2017} Q(m) \\ &\quad = 32 + 9 + 4 \cdot 2 + 2010 \cdot 1 \\ &\quad = 2059, \end{aligned} and the remainder is 59.59.

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