Answer
The difference between the sum of the series and the sum of the first $n$ terms
Work Step by Step
The remainder of an infinite series is the difference between the sum of the series and the sum of the first $n$ terms:
$R_n=\sum_{k=1}^{\infty} a_n-\sum_{k=1}^n a_k=a_{n+1}+a_{n+2}+.....$
