`S_n = n /2*( a_1 + a_n )`

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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, S_n, of the first n terms of an arithmetic series, uses the formula, S_n = n/2(a₁ + a_n).

Input Variables:

a_1: the first term of the sequence

a_n: the last term of the sequence

n: number of terms in the sequence

Output for Formula:

Returns the sum of the arithmetic series