（a^2-b^2)/ab-(ab-b^2)/(ab-a^2)=（a^2-b^2)/ab-b(a-b)/a(b-a)=（a^2-b^2)/ab-(-b)/a=（a^2-b^2)/ab+b/a=（a^2-b^2)/ab+b^2/ab=（a^2-b^2+b^2)/ab=a^2/ab=a/bx/(x^2-y^2)-y/(x^2+2xy+y^2)=x/[(x+y)(x-y)]-y/(x+y)^2...