a^2+b^2+4a-2b+5=0
(a^2+4a+4)+(b^2-2b+1)=0
(a+2)^2+(b-1)^2=0
a=-2 b=1
(a^2+b^2)/2-ab
=(a^2-2ab+b^2)/2
=(a-b)^2/2
=(-2-1)^2/2
=9/2