2008 AIME I 第 6 题

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6.

一个三角形数阵的第一行由奇整数 1,3,5,,991, 3, 5, \ldots, 99 按递增顺序组成。 第一行下面的每一行都比上一行少一个数,最底下一行只有一个数。顶行之后任意一行中的每个数, 都等于它正上方一行中斜上方相邻两个数的和。这个数阵中有多少个数是 6767 的倍数? 13597994812196\scriptsize\begin{array}{ccccccccccc} 1 & & 3 & & 5 & & \cdots & & 97 & & 99 \\ & 4 & & 8 & & 12 & & \cdots & & 196 & \\ & & & & & \vdots & & & & & \end{array}

A triangular array of numbers has a first row consisting of the odd integers 1,3,5,,991, 3, 5, \ldots, 99 in increasing order. Each row below the first has one fewer entry than the row above it, and the bottom row has a single entry. Each entry in any row after the top row equals the sum of the two entries diagonally above it in the row immediately above it. How many entries in the array are multiples of 67?67? 13597994812196\scriptsize\begin{array}{ccccccccccc} 1 & & 3 & & 5 & & \cdots & & 97 & & 99 \\ & 4 & & 8 & & 12 & & \cdots & & 196 & \\ & & & & & \vdots & & & & & \end{array}

答案:17
知识点:整除性数学归纳法奇偶性
难度评级:2600
解答:

用归纳法可得,第 rr 行第 nn 个数为 2r1(r+2n2)2^{r-1}(r + 2n - 2):第 11 行给出 20(2n1)2^0(2n - 1),而第 rr 行相邻两项相加得到 2r1(r+2n2)2^{r-1}(r + 2n - 2) +2r1(r+2n)+ 2^{r-1}(r + 2n) =2r((r+1)+2n2)= 2^r\bigl((r + 1) + 2n - 2\bigr),这正是第 r+1r + 1 行的公式。第 rr 行有 51r51 - r 项,所以 1n51r1 \le n \le 51 - r

因为 6767 是奇数,一个数是 6767 的倍数当且仅当 67r+2n267 \mid r + 2n - 2。当 nn 在第 rr 行中变化时,r+2n2r + 2n - 2 取值为 r,r+2,,100rr, r + 2, \ldots, 100 - r,它们都与 rr 同奇偶且都小于 134134。所以唯一可能的 6767 的倍数是 6767 本身,这要求 rr 为奇数且 r67100rr \le 67 \le 100 - r,即 r33r \le 33

每个奇数行 r=1,3,,33r = 1, 3, \ldots, 33 都恰有一个这样的数,总数为 1717

By induction, the nnth entry of row rr is 2r1(r+2n2):2^{r-1}(r + 2n - 2): row 11 gives 20(2n1),2^0(2n - 1), and summing two adjacent entries of row rr gives 2r1(r+2n2)2^{r-1}(r + 2n - 2) +2r1(r+2n)+ 2^{r-1}(r + 2n) =2r((r+1)+2n2),= 2^r\bigl((r + 1) + 2n - 2\bigr), the formula for row r+1.r + 1. Row rr has 51r51 - r entries, so 1n51r.1 \le n \le 51 - r.

Since 6767 is odd, an entry is a multiple of 6767 exactly when 67r+2n2.67 \mid r + 2n - 2. As nn runs through row r,r, the quantity r+2n2r + 2n - 2 takes the values r,r+2,,100r,r, r + 2, \ldots, 100 - r, all with the same parity as rr and all less than 134.134. So the only possible multiple of 6767 is 6767 itself, which requires rr odd and r67100r,r \le 67 \le 100 - r, that is, r33.r \le 33.

Each odd row r=1,3,,33r = 1, 3, \ldots, 33 contains exactly one such entry, for a total of 17.17.

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