How to prove that?

1. Sep 16, 2006

steven2006

Prove that
$$\sum_{k=0}^n \binom nk 2^k = 3^n, \quad n \in \mathbb{Z}^{+}$$

Can anyone help me? Thanks

2. Sep 16, 2006

StatusX

Do you know the binomial theorem? This is a "yes" or "no, but I looked it up in google and now I do" question.

3. Sep 16, 2006

steven2006

StatusX, thanks.
Sorry for the stupid question! Now i work it out.