The arithmetic series is .

The nth term of an arithmetic sequence is .

Where is first term of the sequence and d is common difference.

Here .

Common difference ,

So d = 7.

The nth term of an arithmetic sequence is .

                 (Substitute )

Distribute terms using distributive property: .

                    (Multiply: )

                          (Subtract: )

Apply addition property of equality: a = b then .

              (Add 1 to each side)

                   (Apply additive inverse property: )

                          (Apply additive identity property: )

                                (Add: )

Apply division property of equality: a = b then .

                           (Divide each side by 7)

                                (Cancel common terms)

                                  (Divide: )

The sum of an arithmetic series .

Where is the first term and is the nth term.

               (Substitute )

                   (Add: )

                         (Multiply: )

                              (Divide: )

The sum of the arithmetic series is 721.