Hi samantha,

Do you mean ?

I think there are many ways you could do this, but the easy way out is to use a cubic polynomial, i.e.

Why? Because we have four values, we can create four equations, and since a cubic polynomial has four unknowns, we can solve for them, integrating our values into the sum... it's hard to explain, but easier if we just do it!

Let

Then, for instance,

Solving either using a calculator or by hand (), we get

So the summation is