Confirm serie formula?

[alkatran]

We just started geometric series ("suite" in french) in math. Anyways, I figured out a formula for calculating the sum of N elements and I came up with:

T(n) = ab^n

S(min, max) = a(b^(max+1)-b^min)/(b-1)
min = first element to sum
max = last element to sum
S(2,3) = ab^2 + ab^3 for example

Confirm?

*edit* fixed a few math errors and put the right equation

[jemidiah]

Would S(2, 4) = ab^2 + ab^3 + ab^4 or ab^2 + ab^4?

[alkatran]

ab^2 + ab^3 + ab^4

[jemidiah]

With S(2, 4)

a(b^(4+1)-b^2)/(b-1) = (ab^5-ab^2)/(b-1) = ab^4-ab-ab^5+ab^2 = -ab^5+ab^4+ab^2-ab <> ab^2 + ab^3 + ab^4

So I'd assume it works with all min-max = 1. Maybe it can be tweaked?

[alkatran]

Maybe that b^min should be b^(min-1) ... *thinks*

Well, I have math class tomorrow to check it over.

[alkatran]

I can't find any situation where the equation doesn't work, it seems fine to me??