2004 AIME II 第 15 题

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15.

一条细长纸带长 10241024 个单位、宽 11 个单位,并被分成 10241024 个单位正方形。纸带反复对折。 第一次折叠时,将纸带右端折到与左端重合并叠在其上,得到一条 51251211、双层厚的纸带。 接着,再把这条纸带的右端折到与左端重合并叠在其上,得到一条 25625611、四层厚的纸带。 这个过程再重复 88 次。最后一次折叠后,纸带变成一叠 10241024 个单位正方形。原来从左数第 942942 个正方形的下面有多少个正方形?

A long thin strip of paper is 10241024 units in length, 11 unit in width, and is divided into 10241024 unit squares. The paper is folded in half repeatedly. For the first fold, the right end of the paper is folded over to coincide with and lie on top of the left end. The result is a 512512 by 11 strip of double thickness. Next, the right end of this strip is folded over to coincide with and lie on top of the left end, resulting in a 256256 by 11 strip of quadruple thickness. This process is repeated 88 more times. After the last fold, the strip has become a stack of 10241024 unit squares. How many of these squares lie below the square that was originally the 942942nd square counting from the left?

答案:593
知识点:折纸过程模拟不变量
难度评级:3500
解答:

经过 ff 次折叠后,纸带长 210f2^{10-f} 个正方形,厚 2f2^f 层。因此从左数的位置 LL 与从右数的位置 RR 满足 L+R=210f+1L + R = 2^{10-f} + 1,从底数的位置 BB 与从顶数的位置 TT 满足 B+T=2f+1B + T = 2^f + 1。右半部分折到左半部分时,左半部分的正方形保持 LLBB 不变;右半部分的正方形被翻转,新的 LL 是旧的 RR,新的 TT 是旧的 BB

942942 个正方形起始为 (L,B)=(942,1)(L, B) = (942, 1)。逐次应用规则,十次折叠后的状态为 (83,2), (83,2), (83,2), (46,15), (19,18), (14,47), (3,82), (3,82), (2,431), (1,594). \begin{gathered} (83, 2),\ (83, 2),\ (83, 2),\ (46, 15),\\ \ (19, 18),\ (14, 47),\ (3, 82),\\ \ (3, 82),\ (2, 431),\ (1, 594). \end{gathered} 例如第四次折叠时纸带长度为 128128,且 L=83>64L = 83 \gt 64,所以新的 LL128+183=46128 + 1 - 83 = 46,新的 TT 是旧的 B=2B = 2,从而 B=16+12=15B = 16 + 1 - 2 = 15

在最后的 10241024 层堆叠中,该正方形从底部数位于第 594594 层,所以其下方有 5941=593594 - 1 = 593 个正方形。

After ff folds the strip is 210f2^{10-f} squares long and 2f2^f layers thick, so the positions LL from the left and RR from the right satisfy L+R=210f+1,L + R = 2^{10-f} + 1, and the positions BB from the bottom and TT from the top satisfy B+T=2f+1.B + T = 2^f + 1. When the right half is folded over onto the left, a square in the left half keeps its LL and B,B, while a square in the right half is flipped: its new LL is its old R,R, and its new TT is its old B.B.

The 942942nd square starts at (L,B)=(942,1).(L, B) = (942, 1). Applying the rule through the ten folds gives (83,2), (83,2), (83,2), (46,15), (19,18), (14,47), (3,82), (3,82), (2,431), (1,594). \begin{gathered} (83, 2),\ (83, 2),\ (83, 2),\ (46, 15),\\ \ (19, 18),\ (14, 47),\ (3, 82),\\ \ (3, 82),\ (2, 431),\ (1, 594). \end{gathered} For example, at the fourth fold the strip has length 128128 and L=83>64,L = 83 \gt 64, so the new LL is 128+183=46128 + 1 - 83 = 46 and the new TT is the old B=2,B = 2, making B=16+12=15.B = 16 + 1 - 2 = 15.

In the final stack of 10241024 squares, this square sits at height 594594 from the bottom, so 5941=593594 - 1 = 593 squares lie below it.

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