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Arithmetic and geometric sequences
Arithmetic sequences
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: an= 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=an-an-1 or .)
n = number of terms
an = nth term
S = sum of the first n terms

 Compute the sum of the first 27 terms of the arithmetic sequence where the first terms are: 3,5,7,9,...

Solution =