# How do you find the sum of the arithmetic sequence having the data given #a_1=81#, #a_n = - 13#, n = 20?

##### 1 Answer

Apr 5, 2016

#### Explanation:

The sum of a series is

where

Substituting in our values of

which is the sum of the series between

If you wanted to find the total sum between

The series clearly has a decreasing common difference if the first is