Possible Duplicate:
Proving ${{n} \choose {r}}={{n-1} \choose {r-1}}+{{n-1} \choose r}$ when $1\leq r\leq n$
I have a dilemma here, how can we show Pascal's Rule :
Show that $ \binom{n}{r} = \binom{n-1}{r-1} + \binom{n-1}{r}, 1 \leq r \leq n. $
I tried solving the right side by substituting everything into the combination's formula but everything gets complicated.. thanks
PS: I tried substituting real valued numbers, and it works, but it should be proof by means of mathematical manipulation.