The sum of an indicated numbers of terms in a sequence is called a Series. The series obtained by adding the terms of an arithmetic progression is called Arithmetic Series.
            For example, the sum of the first seven terms of the sequence is the series,
                       
The above series is also named as the partial sum of the sequence.
            If the numbers of terms in a series is finite, then the series is called a finite series, while a series consisting of an unlimited numbers of terms is termed as an infinite series.

Sum of first n terms of an arithmetic series:
            For any sequence , we have
                       
            If is an A.P., then can be written with usual notation as:
                         --- (1)
            If we write the terms of the series in the reverse order, the sum of terms remains the same, that is
                         --- (2)
            Adding (1) and (2), we get
                       
                               
                               
            Thus,    --- (3)
            If in (3), we replace by its value as , then we obtain another useful rule for  as
                       
                         --- (4)

Example:
            Sum up the following series:
Solution:
            Here, ,
            To find  first, we have
                         
                         
                             
            Now using, we have
                       

Example:
            Find the sum of the first terms of the arithmetic series Also find the sum of the first terms.
Solution:
            Here and
            If is the sum of the first terms, then
                       
                            
                         ------ (1)
            For the sum of first terms, we put  in (1), i.e.