Question 431819
The right hand side is equal to


{{{(n/(n-r))((n-1!)/(n-1-r)!r!)}}}


Since {{{n(n-1)! = n!}}}, and {{{(n-r)(n-1-r)! = (n-r)!}}} the right hand side is equal to


{{{n!/(n-r)!r!}}}, same as the left hand side.