a²－2ab＋2b²＋4a＋8
=a²/2-2ab+2b²+a²/2+4a+8=0
=2(a²/4-ab+b²)+(a²+8a+16)/2
=2(a/2-b)²+(a+4)²/2=0
a/2-b=0
a+4=0
a=-4
b=-2
ab=(-4)*(-2)=8