One formula is like this : f(r) - f(r + 1) and another is f(r) - f(r -1). Sometimes, I see them change to f(r) - f(r + 2), etc...I'm very confused. Which is which and how do I know which one to use for which question? There was one time I used f(r) - f(r + 1) and it was wrong but when I used f(r) - f(r - 1), it was correct. I've forgotten which question because it was on the net. Thanks.