Lesson 3

Turn to page 24 of the textbook to see another way the formula for determining the sum of an arithmetic series can be obtained. Note that there are two versions. Which version matches the formula you derived in Try This 3? Pay attention to how one version can be obtained from the other version.

 

In Try This 3 question 5 you derived a formula to find the sum of the first n terms of an arithmetic series. This formula may have looked similar to this one:

 

sum of the first n terms =

 

This formula can be rearranged to look like this:

 

 

As you read in the textbook, you will discover that another version of this formula is this:

 

a = the first term
n = the number of terms in the series
d = the common difference
tn = the nth term
Sn = the sum of the first n series