Do you have more terms? It might have no elementary relation. If you have more terms, list the difference between each consecutive terms. This forms a new sequence, do the same to this sequence. Keep doing it until the difference is a constant number. If it took you n times to do it, the relation is a polynomial of degree n.
Why don't you post the entire question here without truncating the series? Otherwise, it'll be difficult for the posters here to figure out the answer without even having the complete question to begin with.