Assuming it is an arithmetic series (fixed difference between successive terms) of finite length, you can pair up values: first with last, second to next-to-last, etc., to produce a set of pairs that sum to the same value.

If you have an even number of terms, you have half as many pairs as you had terms. So figure out how to represent that.

(If you have an odd number of terms, you have one term left over and can use essentially the same formula.)