The difference between successive terms is constant, so this is an arithmetic series. The sum is
(number of terms) times (average of all the terms)
In an arithmetic series, the average of all the terms is the average of the first and last. So the sum is
(number of terms) times (average of first and last)
To find the number of terms, find the difference between the first and last terms and divide by the common difference. That tells you how many terms are in the series AFTER THE FIRST TERM, so add 1 to the result to get the number of terms.
