(x2-b2+y2-a2)2-4(ab-xy)2
=(x2-b2+y2-a2)2-(2ab-2xy)2
=(x2-b2+y2-a2+2ab-2xy)×(x2-b2+y2-a2-2ab+2xy)
=[(x-y)2-(a-b)2]×[(x+y)2-(a+b)2]
=(x-y+a-b)×(x-y-a+b)×(x+y+a+b)×(x+y-a-b)