∞∑n=11(n+1)(n+2)(n+3)....(n+k) is equal to
Tn=1(n+1)(n+2)(n+3)....(n+k)
Vn−1=1(n+1)(n+2)(n+3)....(n+k−2)(n+k−1)
Vn−Vn−1=1(n+2)(n+3)....(n+k−1)[1n+k−1n+1]
=1−k(n+1)(n+2)(n+3)....(n+k−1)(n+k)
Vn−Vn−1=(1−k)Tn
∴(1−k)Tn=Vn−Vn−1
(1−k)T1=V1−V0
(1−k)T2=V2−V1
(1−k)T3=V3−V2
...........................
(1−k)Tn=Vn−Vn−1
−−−−−−−−−−−−−−−−−−−
Adding 1−k)Sn=Vn−V0
or(1−k)Sn=1(n+1)(n+2)......(n+k)−11.2.3....k(1−k)Ltn→∞Sn=0−1k!
Ltn→∞Sn=1(k−1)k!