I was doing simple random sampling without replacement and got stuck with this expression. I think on RHS there should be a coefficient of 2 with the second term.
$$( \sum _ { i = 1 } ^ { n } y _ { i } ) ^ { 2 }= \ ( \sum _ { i = 1 } ^ { n } y _ { i } ^ { 2 } + \sum _ { i \neq j = 1 } ^ { n } y _ { i } y _ { j } ) $$
Any help would be much appreciated.