求解prep一题

aj_goodnews

 x & y are integers greater than 1. Is x multiple of y?
(1) 3y^2 + 7y = x
(2) x^2 - x is a multiple of y.

ANS: (1) 3y^2 + 7y = x

y * (3y+7) = x

Clearly, this shows that x is a multiple of y. SUFF

(2) x^2 - x is a multiple of y.

x^2 - x = a*y, where a is a positive integer

x*(x-1) = a*y

Clearly x can be a multiple of y, but can we make an example where x cannot be a multiple of y? Sure! How about x-1 = y? There would be no way for x to be a multiple of y, since y has to be at least 2.

INSUFF

Therefore, A.

i don't really understand, anybody help?

michael_mei

1）3y^2+7y=y(3y+7)=x,所以x一定是y的倍数

2）x^2-x=x(x-1)=my,y可能是x-1的因子而不是x的因子，如x=4,y=3；同时另外一种可能是y是x的因子，如x=6,y=3。所以2）不能得知x是y的倍数

aj_goodnews

以下是引用michael_mei在2008-5-30 22:54:00的发言：

1）3y^2+7y=y(3y+7)=x,所以x一定是y的倍数

2）x^2-x=x(x-1)=my,y可能是x-1的因子而不是x的因子，如x=4,y=3；同时另外一种可能是y是x的因子，如x=6,y=3。所以2）不能得知x是y的倍数

thanx!!!        you rock!!!!

but what does this mean:

 How about x-1 = y? There would be no way for x to be a multiple of y, since y has to be at least 2.

[此贴子已经被作者于2008-5-30 23:56:40编辑过]