This question examines the summation of series. When answering:

(I) directly substituting the value of n according to the conditions to calculate a.

1

、a

2

、a

three

Then, when n≥2 and k≥ 1,

KnCnk=Cn- 1k- 1, and a was obtained by simplifying the sequence

n

=d(d+ 1)

n- 1

, and then discuss the questions in categories to get the answers;