If Sn=∑k=1n ak and limn→∞ an=a, then limn→∞ Sn+1−Sn∑k=1n k is equal to
0
a
2 a
2a
We have,
limn→∞ Sn+1−Sn∑k=1n k=limn→∞ an+1n(n+1)2=0