
Simplifying sigma
Hello I need help with this question:
If f(x) is a funciton of x, defined for all nonnegative integers, simliplify the sum $\displaystyle \sum_{i=1}^n[ (f(i)  f(i1)]$
My attempt: $\displaystyle \sum_{i=1}^n[f(i)] \sum_{i=1}^n]f(i1)]$
Then i kind of expanded f (i1) in wrongly by saying it's equal to f(i)  f(1).
My final answer is wrong because of this assumption.
Where should I go from here?

Re: Simplifying sigma
I'd start by writing out the first few terms of each sum. You might see something cool happen...