若（a+b)的平方+|2b+1|=0,
a+b=0,2b+1=0
b=-1/2,a=1/2
则ab-[2ab-3(ab-1)]
=ab-2ab+3(ab-1)
=ab-2ab+3ab-3
=2ab-3
=2×(-1/2)×1/2-3
=-1/2-3
=-7/2