List the three given terms:                          1   3   7
List the successive differences                        2   4
List the successive diferences of diferences             2

It takes 2 differences to get to a row
containing only one number, so we assume
a polynomial in n of degree 2. 



For n = 1






For n = 1







For n = 3







So we have the system of equations:



Solve that and get A=1, B=-1, C=1

Substituting in:





Edwin