Factorise: x2+y2−z2−2xy
=x2+y2−z2−2xy=x2+y2−2xy−z2=(x−y)2−z2 since (a2−2ab+b2)=(a−b)2 =(x−y+z)(x−y−z) since (a2−b2)=(a+b)(a−b)