$\sum_{k=0}^{n}\frac{C_{n}^{k}}{C_{n+1}^{k+1}}=\frac{1}{2}$
$\frac{C_{n}^{k}}{C_{n+1}^{k+1}}=\frac{n!}{k!(n-k)!}.\frac{(n-k)!(k+1)!}{(n+1)!}=\frac{k+1}{n+1}$
$\sum_{k=0}^{n}\frac{k+1}{n+1}=\frac{(n+1)(n+2)}{2(n+1)}=\frac{n+2}{2}$
chỗ CTSHTQ là $\frac{C_{n}^{k}}{C_{n+k+2}^{k+1}}$ mà bạn