2010 AMC 12A 第 23 题

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所有题目均经美国数学协会(MAA)官方合法授权使用。

23.

90!90! 的最后两个非零数字组成的数等于 nn。求 nn

The number obtained from the last two nonzero digits of 90!90! is equal to n.n. What is n?n?

1212

3232

4848

5252

6868

答案:A
知识点:模运算中国剩余定理末尾零
难度评级:2390
解答:

90!90! 末尾零的个数为 905+9025=21.\left\lfloor\dfrac{90}{5}\right\rfloor+\left\lfloor\dfrac{90}{25}\right\rfloor=21.N=90!1021.N=\dfrac{90!}{10^{21}}.

去掉 1021,10^{21}, 后仍剩下超过两个因数 22,所以 N0(mod4).N\equiv0 \pmod4.

AA90!90! 中不能被 5,5, 整除的因数之积,设 BB 为能被 5.5. 整除的因数之积。每一块 (5j+1)(5j+2)(5j+3)(5j+4)(5j+1)(5j+2)(5j+3)(5j+4) 都满足 241(mod25),24\equiv-1\pmod{25},一共有 1818 块,所以 A1(mod25).A\equiv1\pmod{25}.

B,B, 中去掉 2121 个因数 55 后,剩余因数可分组为 B521=(1234)(6789)(11121314)(161718)(123)1(mod25). \begin{aligned} \dfrac{B}{5^{21}}={}&(1\cdot2\cdot3\cdot4) \\ &\cdot(6\cdot7\cdot8\cdot9) \\ &\cdot(11\cdot12\cdot13\cdot14) \\ &\cdot(16\cdot17\cdot18)(1\cdot2\cdot3) \\ &\equiv-1\pmod{25}. \end{aligned}

因此 90!5211(mod25).\dfrac{90!}{5^{21}}\equiv-1\pmod{25}. 因为 2212(mod25)2^{21}\equiv2\pmod{25},而 222525 的逆元为 13,13,所以 N1312(mod25).N\equiv-13\equiv12\pmod{25}.

同时满足 0(mod4)0\pmod412(mod25)12\pmod{25} 的数模一百为 12(mod100),12\pmod{100},所以最后两个非零数字组成 12.12.

所以正确答案是 A

The number of trailing zeroes in 90!90! is 905+9025=21.\left\lfloor\dfrac{90}{5}\right\rfloor+\left\lfloor\dfrac{90}{25}\right\rfloor=21. Let N=90!1021.N=\dfrac{90!}{10^{21}}.

There are still more than two factors of 22 left after removing 1021,10^{21}, so N0(mod4).N\equiv0 \pmod4.

Let AA be the product of factors of 90!90! not divisible by 5,5, and let BB be the product of the factors divisible by 5.5. Each block (5j+1)(5j+2)(5j+3)(5j+4)(5j+1)(5j+2)(5j+3)(5j+4) is 241(mod25),24\equiv-1\pmod{25}, and there are 1818 blocks, so A1(mod25).A\equiv1\pmod{25}.

After removing the 2121 factors of 55 from B,B, the remaining factors can be grouped as B521=(1234)(6789)(11121314)(161718)(123)1(mod25). \begin{aligned} \dfrac{B}{5^{21}}={}&(1\cdot2\cdot3\cdot4) \\ &\cdot(6\cdot7\cdot8\cdot9) \\ &\cdot(11\cdot12\cdot13\cdot14) \\ &\cdot(16\cdot17\cdot18)(1\cdot2\cdot3) \\ &\equiv-1\pmod{25}. \end{aligned}

Therefore 90!5211(mod25).\dfrac{90!}{5^{21}}\equiv-1\pmod{25}. Since 2212(mod25)2^{21}\equiv2\pmod{25} and the inverse of 22 modulo 2525 is 13,13, we get N1312(mod25).N\equiv-13\equiv12\pmod{25}.

The number congruent to 0(mod4)0\pmod4 and 12(mod25)12\pmod{25} is 12(mod100),12\pmod{100}, so the last two nonzero digits form 12.12.

Thus, A is the correct answer.

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