[gowld]

Neither of those forms is sufficient information to solve a problem, except for a "guess what I'm thinking" puzzle.

You need:

   sum(i = 0 to n) *f*(i)

and you'd like:

   Example: f(0) = *a_0*, f(1) = *a_1*, f(2) = *a_2*

where the stuff in *italics* is given as a concrete substitution.