If a=3+b, thena3-b3 -9ab is :?
a = 3 + 6
a3 – b3 – 9ab
(3 + b)3 – b3 – 9 (3 + b) b
= (3 + b) [ (3 + b)2 – 9b ] – b3
= (3 + b) [ (3)2 + 2(3) (b) + b2 – 9b ] – b3
= (3 + b) [ 9 + 6b + b2 – 9b ] – b3
= (3 + b) (b2 – 3b + 9) – b3
= 3b2 – 9b + 27 + b3 – 3b2 + 9b – b3
= 27