由 小花 » 2008-01-12 09:38
167. 16626-!-item-!-187;#058&010859
If x, y, and z are integers and xy + z is an odd integer, is x an even integer?
Xy+z=odd
兩種可能
Xy=even z=odd
Xy=odd z=even
(1) xy + xz is an even integer.
X=even => Xy=even z=odd=> xy + xz =even => sufficient
X=even => Xy=odd z=even=> xy + z=even 不可能X=even XY =\=odd
X=odd=> Xy=even z=odd=> xy + z=even 不可能
X=odd=> Xy=odd z=even => xy + z=even 不可能
sufficient
(2) y + xz is an odd integer.
X=even=> Xy=even z=odd=>y+xz=odd y=odd時 有可能
X=even=> Xy=odd z=even=> y+xz=odd 不可能 (X=even Xy不可能為odd)
X=odd =>Xy=even z=odd=> y+xz=odd y=even 有可能
X=odd=> Xy=odd z=even=> y+xz=odd 有可能
insufficient
故A
這是我個人的解法
也許其他人有更好的解法