Sume de combinari

[Math_clau]

\(
a) \frac{C_{n}^0}{2}+\frac{C_{n}^1}{3}+.....\frac{C_{n}^n}{n+2}=? \)

\(
b) \frac{C_{n}^0}{1}-\frac{C_{n}^2}{2}+\frac{C_{n}^3}{3}+.....+(-1)^n\frac{C_{n}^n}{n+1}=?
\)