An arithmetic sequence is a sequence of numbers each of which, after the first, is obtained by adding to the preceding number a constant number called the common difference. Thus, 3, 8, 13, 8, 23,... is an arithmetic sequence because each term is obtained by adding 5 to the preceding number.

To solve exercises using aritmetic sequences you need the following formulas:

• The nth term: a_n= a₁ (n-1) d
• The sum of the first n terms:
  where:
  
  a₁ = the first term of the sequence
  d = common difference (Remember you can obtain it using d=a_n-a_n-1 or .)
  n = number of terms
  a_n = nth term
  S = sum of the first n terms