(a+b+c)(a−b−c) = ______
a2+b2−c2−ca
a2−b2+c2−2ca
a2+b2+c2−bc
a2−b2−c2−2bc
(a+b+c)(a−b−c)
=a(a−b−c)+b(a−b−c)+c(a−b−c) =a2−ab−ac+ba−b2−bc+ca−cb−c2 =(a2−b2−c2−2bc) ( ∵ac=ca, bc=cb and ab=ba )
(a+b+c)(a+b−c) = ___.