Hi, I've attached an image of this little snippet from a lecture.

I just do not get why this equality holds. It's probably something to do with factorising etc... but I cannot see why.

X bar is the sum of xi over n (i.e. the mean).

My notation is a bit scruffy, but the summation on the left hand side is on the numerator only (not the entire fraction).

Cannot see why it equals 1!?