4x^（a-1）y与-2/3xy^（-b）是同类项
a-1=1
a=2
1=-b
b=-1
1/2ab²+5/6a²b-1/3ab²-a²b
=1/2ab²-1/3ab²+5/6a²b-a²b
=ab²/6-a²b/6
=ab(b-a)/6
=2*(-1)*(-1-2)/6
=6/6
=1