（1）(a+b)-(c+d)
=a+b-c-d
（2）（a-b)-(c-d)
=a-b-c+d
(3)(a+b)-(-c+d)
=a+b+c-d
(4)-[a-(b-c)]
=-[a-b+c]
=-a+b-c
化简：-4x+3(3分之1x-2)
=-4x+x-6
=-3x-6
0-（x-y-z)
=-x+y+z