（a/ab-b^2 - b/a^2-ab)/(1+a^2+b^2/2ab),其中a=-2,b=1
=[a/b(a-b)-b/a(a-b)]/[(2ab+a²+b²)/2ab]
=[(a²-b²)/ab(a-b)]/[(a+b)²/2ab]
=[(a-b)(a+b)/ab(a-b)]×2ab/(a+b)²
=2/(a+b)
=2/(-2+1)
=-2