标题: 求助PREP一题~~~~~~~~~~~~~~~~~~~~~ [打印本页] 作者: rx_11 时间: 2010-7-16 14:06 标题: 求助PREP一题~~~~~~~~~~~~~~~~~~~~~ if x and y are positive integers, is x an even integer? (1) x(y+5) is an even integer. (2) 6y^2+41y+25 is an even integer 答案是E。。不解···求解答~~~作者: youngerlee 时间: 2010-7-16 14:18
with following steps, I get an E answer.....
if x and y are positive integers, is x an even integer? (1) x(y+5)=even (2) 6y^2+41y+25=even
[1]x(y+5)=even
when y is even, y+5 is an odd, then x should be even. then y is odd, y+5 is even, we have no idea about x in this case.
insufficient.
[2]6y^2+41y+25=even
25 odd, so 6y^2+41y should be odd AND SHOULD NOT BE 0
6y^2+41y should be odd means y[6y+41]should be odd
so y and [6y+41] should be all odds
when y even, 6y+41 odd y[6y+41] even when y odd, 6y+41 odd y[6y+41] odd
then, y must be odd
take 'y=odd' into factor [1] when y odd, x have no idea