首页
登录
职称英语
Which of the following cannot be expressed as the sum of two prime numbers?A、21
Which of the following cannot be expressed as the sum of two prime numbers?A、21
游客
2024-01-13
46
管理
问题
Which of the following cannot be expressed as the sum of two prime numbers?
选项
A、21
B、14
C、18
D、28
E、23
答案
E
解析
这道题目用排除法当然可以迅速解出来,比如:21=19+2,14=7+7,18=13+5,28=23+5,只剩下23,当然选择E。但是如果利用上面的定理,则速度更快。五个选项中B、C、D都为大于2的偶数,根据定理必然可以表达为两个质数的和;剩下选项A、E均为奇数,若两数相加为奇数,则这两数必然为一奇一偶。而这个数又要能够表达为两个质数的和,那么必须这一奇一偶都为质数,我们知道在所有质数中2是唯一的一个偶数,因此若A或E能够表达为两质数的和,其中一个一定为2,选项A:21-2=19,E:23-2=21,19为质数,21=3×7不为质数,因此E是正确答案。
转载请注明原文地址:https://tihaiku.com/zcyy/3359962.html
相关试题推荐
Notalways______theywantto.A、peoplecandowhatB、peoplecannotdowhatC、can
Thegovernmentplaced______onthenumbersofforeigncarsthatcouldbeimported
Notalways()theywantto.A、peoplecandowhatB、peoplecannotdowhatC、canpeo
Icannottellthe()differencebetweenthetwins.A、slenderB、singleC、simpleD、s
Notalways______theywantto.A、peoplecandowhatB、peoplecannotdowhatC、can
Notalways______theywantto.A、peoplecandowhatB、peoplecannotdowhatC、can
Itwouldbe______arisktoletthechildgotoschoolbyhimself.A、followingB、p
Mancannotgoonincreasinghisnumberatthepresentrate.Inthenext30ye
Mancannotgoonincreasinghisnumberatthepresentrate.Inthenext30ye
Mancannotgoonincreasinghisnumberatthepresentrate.Inthenext30ye
随机试题
Keepingafullsocialcalendarmayhelpprotectyoufromdementia(痴呆症),res
Womenknowthatalackofsleepleavesthemirritable.However,mostofthem
这些石窟刻于距离敦煌东南方向25公里处鸣沙山的悬崖上。Thesecaveswerecutonacliff25kilometerssouthea
工程量清单的编制阶段是在:()A.设计方案确定后 B.初步设计完成后 C
工程量清单漏项或设计变更引起的新的工程量清单项目,其相应综合单价由()提出,经
数字信号B=1时,图示两种基本门的输出分别为:
女性,28岁,妊娠36周,既往体健,化验血Hb60g/L,MCV102fl,WB
鼻咽癌好发于鼻咽顶前壁及咽隐窝。
左边给定的是纸盒的外表面,下列哪一项能由它折叠而成: A.如上图 B.如
通过使收益现值与成本相等来求出利率或贴现率的值,然后再将这种收益率与其他投资的报
最新回复
(
0
)