原式=x^2-4xy+4y^2-2y^2=(x-2y)^2-(√2y)^2=(x-2y+√2y)(x-2y-√2y)
= x^2 -4xy +4y^2 -2y^2=(x-2y)^2- 2y^2=(x-2y+√2y)(x-2y+√2y)
（x-√2y）^
