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, Sn = n/2(a1 + an).
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