183. DS: 6 of n's factors are positive integer, 2 of n's 6 factors are prime，n=? 1)忘了......... 2) n<15 狗主人补充：我选的B， n =12吧 思路： 关于条件二，我也没想出简单方法，一个一个算吧，可以算出12的。。。 希望那个同学们能给出新的提醒。确实是12，支持狗主人说法。
DS: 6 of n's factors are positive integer, 2 of n's 6 factors are prime，n=? 1)忘了......... 2) n<15
Since n has 6 factors, n is not a square of a natural number because all of its factors are in pairs. Since n has two prime factors, then n = (x^a)*(y^b), wherein both x and y are prime; (a+1)(b+1)=6=2*3 So a = 1; b = 2
If n<15 = 3 * 5, then the prime factors for n can only be (2, 3), (2, 5) wherein a = 1; b = 2. Then only 2^2*3, the smallest possible n is smaller than 15. So n = 12.
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发表于 2010-12-10 17:55:08
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