I am studying generating functions in combinatorics, and came across a problem that has already been posted here:
Generating function and combinatorics
=x^10(1-x^6)^10 * (1+x+x^2....)^10
I simply don't understand why the answer to this is:
C(10+15-1,15) - 10*C(10+9-1,9) + C(10,2)*C(10+3-1,3)
I can understand the first two combinations, but where did the C(10,2)*C(10+3-1,3) come from? I would appreciate any help to understand this. Thanks!