(a+b+c)(a+b−c) = ___.
a2+2ab-c2
a2+4ab+b2-c2
a2+2ab+b2
a2+2ab+b2-c2
(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+2ab+b2−c2) ( ∵ac=ca and bc=cb )
(a+b+c)(a−b−c) = ______
(a+b+c)(a+b-c) = ___.