Solution:
Given expression is
Now, coefficient of x18 in the product
=coefficient of y18 in the product
=coefficient of y18 in (1 - x3 )9- coefficient of x16
in (1 - x3)9
Since, (r + 1)th term in the expansion of
Now, for x18, 3r = 18r = 6
and for x18 ,
and for x16,
.'. Required coefficient is