Given that a (A-1) - (A & # 178; - b) = 4, find the value of algebraic formula 2 / A & # 178; + B & # 178; - ab

Given that a (A-1) - (A & # 178; - b) = 4, find the value of algebraic formula 2 / A & # 178; + B & # 178; - ab

a(a-1)-(a²-b)=4
a²-a-a²+b=4
b-a=4
Then:
[(a²+b²)/2]-ab
=[(a²+b²)-2ab]/2
=[a²-2ab+b²]/2
=(a-b)²/2
=(b-a)²/2
=8