Školjka

Dokaži kombinatornim argumentom: \begin{enumerate} \item $$\binom{n}{k} = \binom{n}{n-k}$$ \item $$\sum_{k=0}^n \binom{n}{k} = 2^n$$ \item $$\binom{n}{k} + \binom{n}{k-1} = \binom{n+1}{k}$$ \end{enumerate}