If we use addition or subtraction operators between the terms of a sequence, we get a series. 

\({t_1} + {t_2} + {t_3} + {\rm{ }} \ldots  \ldots .{\rm{ }} + {t_n}\) is called a series.

A series is called a finite series or an infinite series according as the number of terms in a series is finite or infinite.

For example 1 + 2 + 3 + 4 + ….. is a series that increases by a constant difference of 1. Similarly the series \(1 + \frac{1}{2} + \frac{1}{4} + \frac{1}{8} + ....\) decreases contin-uously and each term is half of the previous term. In general nth term of series is denoted by \({t_n}\) and sum up to n terms is denoted by \(\sum {{t_n}} \).