x3 - 3xy2 - 2y3
= x3 - xy2 - 2xy2 - 2y3
= (x3 - xy2) - (2xy2 + 2y3)
= x . (x2 - y2) - 2y2 . (x + y)
= x . (x - y) . (x + y) - 2y2 . (x + y)
= (x + y) . [x . (x - y) - 2y2]
= (x + y) . (x2 - xy - 2y2)
= (x + y) . (x2 - 2xy + xy - 2y2)
= (x + y) . [(x2 - 2xy) + (xy - 2y2)]
= (x + y) . [x . (x - 2y) + y . (x - 2y)]
= (x + y) . (x - 2y) . (x + y)
= (x + y)2 . (x - 2y)