(a+b)的平方+l2b-1l=0
那么a+b=0 2b-1=0
解得:a= -1/2 b=1/2
ab-{2ab-3(ab-1)}
=ab-2ab+3ab-3
=2ab-3
=2×1/2×(-1/2)-3
= -1/2-3
= -3.5