Excel's tendency curves: How to...?

Given a series of points, is easy to make Excel to graph a curve showing the tendency in terms of a certain math model, and the correlation coefficient showing how good is the fitness... Up to here everything is fine.
But here comes my foolish question: how can I take the parameters of that curve to make further calculations with them? I mean, I can just take the values I see and write them manually in the cells, but if any of the data changes, that change is not reflected automatically, and so spreadsheet won't work the way I expect.
Please find attached a sample file. The original XLS extension was changed to TXT just to make the attachment possible, as I saw on another post when searching for a similar question; please change it to XLS before.
Best regards,
CPet