Factorise:
(a2−1)(b2−1)+4ab
(a2−1)(b2−1)+4ab=a2b2−a2−b2+1+4ab=(a2b2+2ab+1)−(a2−2ab+b2)=(ab+1)2−(a−b)2=[(ab+1)+(a−b)][(ab+1)−(a−b)]=(ab−a+b+1)(ab+a−b−1)
Factorise: a2+ab(b+1)+b3
a2−81(b−c)2
a2−(2a+3b)2