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标题: 请教数学 PREP Q183 [打印本页]

作者: WesternCder 时间: 2010-12-31 06:59 标题: 请教数学 PREP Q183

If m is a positive oddinteger between 2 and 30, then m is divisible by how many different positiveprime numbers?

(1) m is not divisible by 3.
(2) m is not divisible by 5.【答案】A
为什么答案是A呢？

作者: piacia 时间: 2011-1-1 06:59

3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27

(1) m is not divisible by 3.
5, 7, 11, 13, 17, 19, 23, 25
Each one only has one prime number as its factor. So condition 1 is

sufficient.

(2) m is not divisible by 5.
3, 7, 9, 11, 13, 17, 19, 21, 23, 27
21= 3 * 7. So 21 has two prime numbers as factors while the rest of the

numbers only have one prime number as their factor. So condition 2 is not

sufficient.

作者: WesternCder 时间: 2011-1-1 21:12

明白了，多谢！

作者: zm_patrickS 时间: 2011-1-2 07:01

对啊对啊刚刚没想到 25不是prime

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