(999)3= (1000 - 1)3= (1000)3 - (1)3 - 3(1000)(1) (1000 - 1)= 1000 000 000 - 1 - 3000 × 999= 1000 000 000 - 1 - 2997000= 997002999
15. Factorise:a2 + b2 - 2 (ab - ac + bc)
Factorise:(x - 3y)3 + (3y - 7z + (7z - x)3