∵   x4 - (y + z)4 = [x2]2 - [(y + z)2]2                       = [(x2) + (y + z)2] [(x2) - (y + z)2]                                                                                                      [Using a2 - b2 = (a+b)(a-b)]We can factorise [x2 - (y + z)2] further as       x2 - (y + z)2 = [(x) + (y + z)][(x) - (y + z)]                         = (x + y + z)(x - y - z)
∴      x4 - (y + z)4 = (x + y + z)(x - y - z)[x2 + (y + z)2]