An arithmetic sequence is a sequence in which, beginning with the second term, each term is found by adding the same value to the previous term. Its general term is described by
a _{n} = a _{1} + ( n –1) d
The number d is called the common difference. It can be found by taking any term in the sequence and subtracting its preceding term.
Example 1
Find the common difference in each of the following arithmetic sequences. Then express each sequence in the form a _{n} = a _{1} + (n – 1) d and find the twentieth term of the sequence.

1, 5, 9, 13, 17, …

…

1, 5, 9, 13, 17, …
Therefore, the twentieth term of the sequence is
a _{20} = 1 + (20 – 1)4 = 1 + 76 = 77

…
Since
Then
Therefore, the twentieth term of the sequence is