x^2-2xy-8y^2=x^2-2xy+y^2-9y^2=(x-y)^2-(3y)^2=(x-y+3y)(x-y-3y)=(x+2y)(x-4y)
x^2-2xy-8y^2=(x-4y)(x+2y)
（x+2y）*(x-4y)
(x+2y)(x-4y)
