1. The problem statement, all variables and given/known data 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. 2. Relevant equations NA (i dont think there are any equations for this) 3. 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.