Just as we studied special types of sequences, we will look at special types of series. Recall that an arithmetic sequence is a sequence in which the difference between any two consecutive terms is the common difference, . The sum of the terms of an arithmetic sequence is called an arithmetic series. We can write the sum of the first terms of an arithmetic series as:
We can also reverse the order of the terms and write the sum as
If we add these two expressions for the sum of the first terms of an arithmetic series, we can derive a formula for the sum of the first terms of any arithmetic series.
Because there are terms in the series, we can simplify this sum to
We divide by 2 to find the formula for the sum of the first terms of an arithmetic series.
A General Note: Formula for the Sum of the First n Terms of an Arithmetic Series
An arithmetic series is the sum of the terms of an arithmetic sequence. The formula for the sum of the first terms of an arithmetic sequence is
How To: Given terms of an arithmetic series, find the sum of the first terms.
- Identify and .
- Determine .
- Substitute values for , and into the formula .
- Simplify to find .
Example 2: Finding the First n Terms of an Arithmetic Series
Find the sum of each arithmetic series.
Solution
- We are given and .Count the number of terms in the sequence to find .
Substitute values for and into the formula and simplify.
- We are given and .Use the formula for the general term of an arithmetic sequence to find .
Substitute values for into the formula and simplify.
- To find , substitute into the given explicit formula.
We are given that . To find , substitute into the given explicit formula.
Substitute values for , and into the formula and simplify.
Use the formula to find the sum of each arithmetic series.
Example 3: Solving Application Problems with Arithmetic Series
On the Sunday after a minor surgery, a woman is able to walk a half-mile. Each Sunday, she walks an additional quarter-mile. After 8 weeks, what will be the total number of miles she has walked?
Solution
This problem can be modeled by an arithmetic series with and . We are looking for the total number of miles walked after 8 weeks, so we know that , and we are looking for . To find , we can use the explicit formula for an arithmetic sequence.
We can now use the formula for arithmetic series.
She will have walked a total of 11 miles.
Try It 5
A man earns $100 in the first week of June. Each week, he earns $12.50 more than the previous week. After 12 weeks, how much has he earned?
Candela Citations
- Precalculus. Authored by: OpenStax College. Provided by: OpenStax. Located at: http://cnx.org/contents/fd53eae1-fa23-47c7-bb1b-972349835c3c@5.175:1/Preface. License: CC BY: Attribution