An arithmetic sequence or progression, is a sequence where each term is calculated by adding a fixed amount to the previous term.
Arithmetic Sequences
This fixed amount is called the common difference, d. It can be positive or negative.
The common difference can be calculated by subtracting a term from the one following it.
e.g. Common difference = t_{ n+1} − t_{ n}
The first term of an arithmetic sequence is shown by the variable a.
Arithmetic Sequence

First term, a

Common difference, d

2, 6, 10, 14, ...

2

6 − 2 = 4 
20, 14, 8, 2, ...

20

14 − 20 = 6
8 − 14 = 6 etc. 
General Term, t_{n}
An arithmetic sequence can be written:
First term

Second term

Third term

Fourth term

General term (n th term)


t _{1}

t _{2}

t _{3}

t _{4}

...

t_{n}

a

a + d

a + 2d

a + 3d

...

a + (n − 1)d

Example 1 
What is the common difference of the arithmetic sequence: 3.5, 8.3, 13.1, 17.9, ... 
Common difference = t_{ n+1} − t_{ n} Second term − first term = t_{2} − t_{1} = 8.3 − 3.5 = 4.8 Check: Third term − second term = 13.1 − 8.3 = 4.8 The common difference is 4.8 
Example 2 
Find the 20th term of the arithmetic sequence: 3, 9, 15, 21, ... 
Common difference, d = 9 − 3 = 6 using t_{n} = a + (n − 1)d t_{20} = 3 + (20 − 1)6 The 20th term is 117 
Example 3  Which term of the sequence 12, 15, 18, .. would be equal to 54? 
Common difference, d = 3 Using t_{n} = a + (n − 1)d 54 = 12 + (n − 1)3 The 15th term would be 54 
Example 4 
The third term of an arithmetic sequence is 18 and the tenthterm is 74. Find the first term, a, and the common difference, d, and thus list the first four terms of the sequence. 
t _{3} = 18 using t_{n} = a + (n − 1)d 18 = a + (3 − 1)d 18 = a + 2d 56 = 7d ( subtracting) 18 = a + 2 x 8 The sequence is 2, 10, 18, 26, ... 
Arithmetic Series
If terms of an arithmetic sequence are added together an arithmetic series is formed.
2 + 4 + 6 + 8 is a finite arithmetic series
2 + 4 + 6 + 8 + ... is an infinte arithmetic series
To find the sum of the first n terms of an arithmetic sequence use the formula:
Sum of first n terms of arithmetic sequence d = common difference 
OR
An equivalent formula involving the last term, l
Sum of first n terms of arithmetic sequence 
Example 1

What is the sum of the first 15 terms of the arithmetic sequence: 3, 6, 9, 12, ... 


Example 2

The first term of an arithmetic sequence is 5 and the last term is 250. The sum of this series is 1020. How many terms does it have? 


Example 3

An athlete does 20 pressups on the first day of a training routine. On the second day she does 24 pressups and on the third day 28 pressups. If she follows this pattern for 30 days, how many pressup will she have done altogether? 
