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Maths Exercices
Arithmetic series
An arithmetic series is the sum of an aritmetic sequence. That is,


The series 3 + 7 + 11 + 15 + 19 + ... + 99 is an aritmetic series which first term is a1=3 and the common difference is d=4.

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: S_n=(a_1+a_n)/2•n
    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
    Sn = sum of the first n terms
Sometimes we use:

Find the sum of the first 40 terms, s40 of 3, 7, 11, 15,...

Compute the sum of the first 20 terms of the arithmetic sequence where a2 = -7 and a6 = -23

Solution =