[(x^4-y^4)*(2x^2+xy-y^2)]÷[(x^2+y^2)/(2x^2+xy-y^2)]
=[(x^2+y^2)(x^2-y^2)(x+y)(2x-y)]×[(x+y)(2x-y)/(x^2+y^2)]
=x^2-y^2