prep math DS

rainbowmanutd · 发表于 2012-4-12 06:43:41

167.
If x, y,and z are integers and xy + z is an odd integer, is x an even integer?

(1) xy+ xz is an even integer.

(2) y +xz is an odd integer.
--------------------------------------------------------------------------------------------------------------------------------------------------------【答案】A

【思路】xy + z = odd integer ,xy(odd)+z(even)= odd integer , xy(even)+z(odd)=odd integer
(1) xy+ xz = even , xy(even)+xz(even)=even , so z 應該為odd , x為even , xz才為even , xy(odd)+xz(odd)=even ,
z 為even , xz不等於odd , 充份
(2) y + xz = odd integer ,y(odd)+xz(even)=odd , z=even ,x=even , y(even)+xz(odd)=odd ,z=odd,x=odd , 所以不充份

这个又没有简单的方法呢？解释看的太晕了??

prep math DS

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