(x-2y)^2-(x+2y)^2+2x^2+8y^2
=x^2-4xy+4y^2-x^2-4xy-4y^2+2x^2+8y^2
=2x^2-8xy+8y^2
=2（x^2-4xy+4y^2）
=2（x-2y）²