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√(79)(80)(81)(82)
6400
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当前版本由 191676xhtih 更新于2023-08-07 21:07:19 感谢由 191676xhtih 对此题目的解答所做出的贡献。
6400=sqrt(80*80*80*80)
選項A為sqrt(79*80*81*82)
当前版本由 USCIS 更新于2021-07-26 14:41:18 感谢由 USCIS 对此题目的解答所做出的贡献。
也可以这么算： a是79*80*81*82 b可以当作80^4 第一步都除80: a. 79*81*82 b. 80*80*80 第二步把80当成x：a. (x-1)(x+1)(x+2) 和 b. x^3 比大小 a可以化作 x^3+2x^2-x-2 那就是 2x^2-x-2 和 0比大小 很显然80足够大到把这个数字变正所以a大
当前版本由 294302eshdi 更新于2020-11-06 17:39:16 感谢由 294302eshdi 对此题目的解答所做出的贡献。
(79)(80)(81)(82)=(80.5-1.5)(80.5-0.5)(80.5+0.5)(80.5+1.5)=(80.5^2-1.5^2)(80.5^2-0.5^2)
且(80.5^2-1.5^2)<(80.5^2-0.5^2)
(79)(80)(81)(82)开根号后得到的设X，(80.5^2-0.5^2)>X>(80.5^2-1.5^2)，即6480>X>6478，所以选A
当然可以直接计算器了，更快。只是提供多一种思考方式
当前版本由 更新于2019-07-11 15:33:02 感谢由 对此题目的解答所做出的贡献。
80.5^2-1＞6400
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没错啊 6400=80✖️80，你要两个都带根号才能算，所以6400=√80 80 80 80
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