prep的一道数学题，大家帮帮忙=。=！！！！

flyfairy019

The positive integer k has exactly two positive prime factors, 3 and 7. If k has a total of 6 positive factors, including 1 and k, what is the value of k?

1) 3^2 is a factor of k.
2) 7^2 is NOT a factor of k

答案是D，跪求好心人解释....

angelsgx

k= 3^n * 7^m

(n+1)(m+1)=6

n=2, m=1
or
m=2, n=1

1) n=2, then you can get k
2) m=/=2, then you can still get the value of k, which is the same as the one you get from 1)

flyfairy019

谢谢~回答的很清楚~

sbjqbS

这题我看了半天。。先假设了42。。发现不对。。又假设了63。。然后找到规律了。。
总之就是2个3 或 2个7嘛 只有2个选择。。第一个条件给了一种。。第二个条件排除一种