若(a-o.5)的二次方+2b+1的绝对值=o,则ab-[2ab-3(ab-1)]的值是多少

若(a-o.5)的二次方+2b+1的绝对值=o
a-0.5=0
a=0.5
2b+1=0
b=-0.5
ab-[2ab-3(ab-1)]
=ab+ab-3
=2ab-3
=-0.5-3
=-3.5