`S_n = n /2*(2* a_1 +( n -1)* d )`

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The indicated sum of the terms of a sequence is called a series. An arithmetic series is the sum of the terms in the corresponding arithmetic sequence. To find the sum, Sn, of the first n terms of an arithmetic series, uses the formula, S_n = n/2(2a₁ + (n-1)d).

Input Variables:

a_1: the first term of the sequence
 d: common difference (Subtracting any term from the term that follows it always results in the same number)
 n: number of terms in the sequence

Output for Formula:
 Returns the sum of the arithmetic series