You can put this solution on YOUR website! Consider the way that Gauss showed us how to sum an arithmetic sequence.
Your example is an arithmetic sequence so there is some number d that you add to each term to obtain the next term.
The middle term of a term sequence is ; so, this is the th term.
The last term is terms further along in the sequence so it is .
Likewise the first term is terms before the th term so it is .
Thus the sum of the first and last term is
now use Gauss' technique to find the sum of the sequence
where =number of terms, =first term, and =last term
