I saw this in slides for a economics class but I cannot see how to prove it using this hint - they seem to all cancel out regardless of how I distribute the summation:
$ \frac{1}{N} \Sigma ab = \frac{1}{N} \Sigma a \frac{1}{N} \Sigma b = \bar a \bar b $
Prove by writing:
$ a = \bar a + a - \bar a $,
$ b = \bar b + b - \bar b $
