Thanks for taking the time to look at this, it's going to be one of those problems where I'm either missing something really obvious, or there's no simple way to approach it, so I thought I'd post it up on see if anyone has any ideas:

From this summation:

I want to isolate

, since I have a substitution for it.

My question is then, is there a simple way to do this? The only way I can see is to sum each part in turn, multiply the 2 together, and then manually subtract the unwanted terms this also spews forth.

Any help or thoughts would be appreciated, thanks again.