[(a+b)/2]²-[(a-b)/2]²
={[(a+b)/2]+[(a-b)/2]}{[(a+b)/2]-[(a-b)/2]}
=(a/2+b/2+a/2-b/2)(a/2+b2-a/2+b/2)
=a*b
=(-1/8)*2
=-1/4