一个DS请教

lala

Is 30 a factor of N?
2)        30 is a factor of N^2 ;  2) 30 is a factor of 2N

这是费费逻辑宝典中的一个题。答案选A。 但我觉得C才对。关键是：题目没说N是否为整数，可否把N理解成无理数（如根号30）？

uclb5550

N must be integer, otherwise, the answer will be E.

30=2*3*5  30 is factor of N^2, which means N must be 30 at least.
2*3*5*M=N^2  in order to set up the equation, M must be 2*3*5 at least. therefore, the conclusion is out here. 30 must a factor of N.

clear?

lala

I don't understand your first  sencence.
到底这道题，答案是哪个？

uclb5550

answer is A. sorry, my words makes some perplexity for you. what i mean is, if as your speculation puts it--N is irrational number, the answer would be E.

by the way, the reason why B is wrong is the N can be 15.