Generalized formula for sum of products. Q:The sum of all possible products of the first n natural numbers taken two by two   is?
 I did not understand the question as it is.What exactly is being asked?I'd really appreciate an answer explaining what is being asked along with a solution.

It is asked for $\displaystyle\sum\limits_{1\le i<j\le n} i\cdot j$.
Hint: Think of $(1+2+\dots+n)^2$.
