①x^4-4x^3y+4x^2y^2 =x^2(x^2-4xy+4y^2) =x^2(x-2y)^2 ②(2x+3y)^2-(2x-y)^2 =[(2x+3y)+(2x-y)][(2x+3y)-(2x-y)] =(4x+2y)(4y) =8y(2x+y)
