Question 21401
LHS=nCr=n!/[(n-r)!r!]
RHS=(n/r)(n-1)C(r-1)=(n/r)[(n-1)!/{(n-1)-(r-1)}!(r-1)!]
=[n(n-1)!/{(n-1-r+1)!(r-1)!r]=n!/[(n-r)!r!]=LHS