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## Homework Statement

Sorry if the question sounds a bit off, i wasnt quite sure how to word it.

My math question is: given the expression Sn=n^2 + 2n, determine an expression, in simplified form, for Sn-1 in terms of n.

## Homework Equations

NA (i dont think there are any equations for this)

## The Attempt at a Solution

I honestly dont know how to go about this properly but here's what ive done:

First three terms are 3, 8 and 15. All of which have a different 'common ratio' when 8 is divided by 3, its 2.67 but when 15/8 its 1.875?

Which confuses me further, and as for the original question, no idea how to proceed.

Any help is much appreciated, thanks.