【答案解析】提示：只要证明[-Y] _补 =-[Y] _补 ，上式即得证。
因为[X+Y] _补 =[X] _补 +[Y] _补 (mod 2)
令 Y=-X
[0] _补 =[X] _补 +[-X] _补
所以[-X] _补 =-[X] _补 (mod 2)
即[X-Y] _补 =[X] _补 -[Y] _补 =[X] _补 +[-Y] _补