(1).x^2-8xy+7y^2 =（x-y)(x-7y)
(2).4x^2+9xy+2y^2 =(4x+y)(x+2y)
(3).2x^2+7xy+3y^2 =(2x+y)(x+3y)
(4).6x^2+xy-2y^2=(2x-y)(3x+2y)