原式=(a^2-2ab+b^2)-(a^2+2ab+b^2) =a^2-2ab+b^2-a^2-2ab-b^2 =-4ab
(a-b)^2-(a+b)^2=(a-b-a-b)(a-b+a+b)=(-2b)(2a)=-4ab
(a-b)^2-(a+b)^2=(a-b+a+b)(a-b-a-b)=-4ab