Chapter 10 - Section 10.2 - Arithmetic Sequences - Exercise Set - Page 1061: 72

See below.

An arithmetic sequence is defined as an array of numbers which consists of a common difference between every two consecutive terms. For example: $1, 3,5, 7,....$ Here, the common difference is $d=3-1=2$ The sum of an arithmetic sequence is given by: $S_n=\dfrac{n}{2}[a_1+a_n]$ and the nth term for an arithmetic sequence is given by $a_n=a_1+(n-1) d$