2(a-2)^2+(b+1)^2=0,
a-2=0 a=2
b+1=0 b=-1
3a^2 b+ab^2-3a^2 b+5ab +ab^2-4ab+1/2a^2b
=1/2a^2b+2ab^2+ab
=1/2x4x-1+2x2x1+2x-1
=-2+4-2
=0