Simplifying product of differences
Why didn't you even try for small numbers? If you had, you could have canceled out all the terms. ;)
The expression turns out to be
$\dfrac{2}{3}.\dfrac{3}{4}.\dfrac{4}{5} \dots\dfrac{n-2}{n-1}.\dfrac{n-1}{n}$
Cancel out the terms and you're left with just $\frac{2}{n}$.