In a linear series there must be a certain difference between the terms.
When you subtract or proportion any term from the term immediately preceding it, the constant you get will be the common difference of the linear series. In the series given in the question, the common difference is 

Therefore, the general formula of the series must contain the expression 
 Since the first term is
 Since the first term is 
 , we will add
, we will add 
 .
.
Answer: 
