Answer
The sum of an arithmetic sequence is given by: $S_n=\dfrac{n}{2}[a_1+a_n]$.
See below.
Work Step by Step
The sum of an arithmetic sequence is given by: $S_n=\dfrac{n}{2}[a_1+a_n]$ and the nth term for an arithmetic sequence is given by $a_n=a_1+(n-1) d$
Here, $a_1 \to$ Initial Term and $d=$ Common difference.
The sum of the first n term can be obtained by taking the half of the product between the number and adding the initial term and last term of the arithmetic sequence.
