Hi there. I have some doubts with this exercise. I have to make the summation for:

I've solved this way:

The doubt I've got is about the "negative" part of the summation, I know how to solve the geometric progression, but I'm not sure about the other part. I've proceeded this way:

This is what I did:

I'm not sure about this, may I should do it this other way?:

I mean, taking the values of the sequence for the series for every "i", and making that summation but the accumulation? cause I think it has not much sense what I did before, I mean there is no accumulation for the negative part, right?

Bye there, and thanks for your help.