$\sum_{k=0}^{20} \left({}^{20}C_{k}\right)^{2}$ is equal to :

If $(1 + x)^n = C_0 + C_1x + C_2x^2 + ... + C_nx^n$,then the value of $C_0 + C_2 + C_4 + C_6 + ...$ is

$\binom{47}{4} + \sum_{r=1}^5 \binom{52-r}{3} = \dots$

The sum to $(n + 1)$ terms of the series $\frac{C_0}{2} - \frac{C_1}{3} + \frac{C_2}{4} - \frac{C_3}{5} + \dots$ is

If $n$ is a positive integer such that $n \ge 3$,then the value of the sum to $n$ terms of the series $1 \cdot n - \frac{(n - 1)}{1!} (n - 1) + \frac{(n - 1)(n - 2)}{2!} (n - 2) - \frac{(n - 1)(n - 2)(n - 3)}{3!} (n - 3) + \dots$ is:

$3 \cdot C_0 + 7 \cdot C_1 + 11 \cdot C_2 + \ldots + (3 + 4n) C_n =$