TITLE: Proof of the identity $2^n = \sum\limits_{k=0}^n 2^{-k} \binom{n+k}{k}$
QUESTION [15 upvotes]: I just found this identity but without any proof, could you just give me an hint how I could prove it?
$$2^n = \sum\limits_{k=0}^n 2^{-k} \cdot \binom{n+k}{k}$$
I know that $$2^n = \sum\limits_{k=0}^n \binom{n}{k}$$ but that didn't help me

REPLY [2 votes]: $$\sum\limits_{k=0}^{n}\binom{k+m}{m}z^k=\frac{1}{(1-z)^{m+1}}-\frac{z^{n+1}}{(1-z)^{m+1}}\sum\limits_{k=0}^{m}\binom{n+k}{k}(1-z)^k$$
$$n=m, z=\frac{1}{2}$$
$$\sum\limits_{k=0}^{n}\binom{k+n}{n}\frac{1}{2^k}=\frac{1}{\frac{1}{2^{n+1}}}-\frac{\frac{1}{2^{n+1}}}{\frac{1}{2^{n+1}}}\sum\limits_{k=0}^{n}\binom{n+k}{k}\frac{1}{2^k}$$
$$S=2^{n+1}-S, 2S = 2^{n+1}, S=2^n$$