21.1 Arithmetic sequences
Arithmetic sequences
An arithmetic sequence (arithmetic progression) is a number sequence with a common difference between successive terms. By using the arithmetic sequence formula, we can easily find the value of a term and the common difference in the sequence.
Basic concepts:
 Sigma notation
Lessons
Notes:
• arithmetic sequence: a sequence with a common difference between successive terms
• The nth term, ${t_n}$ ,of an arithmetic sequence:
${t_n} = {t_1} + \left( {n  1} \right)d$
where, ${t_n}$: nth term
${t_1}$: first term
$d$ : common difference

1.
Arithmetic sequence formula
Consider the arithmetic sequence: 5, 9, 13, 17, … . 
2.
Determine $t_1,d,t_n$ for the sequences in which two terms are given