It's been a while since I've seen this type of equation, not sure what to do here:
I am not sure this even makes sense, because if i = 1 then why not just say X subscript 1, instead of X subscript i? Also, "n-1" implies that we're considering a sample set of "n" numbers, which implies that the value of "i" changes "n" amount of times.
I don't know for sure though.
Hope you will help.