On completion of this (finishing)

1. 求和\[ \sum_{k=0}^n k {n \choose k} \]
   解法：\[ \begin{align} (x = 1 时)\sum_{k=0}^n k {n \choose k} & = \sum_{k=0}^n {n \choose k}\frac{\partial}{\partial x} x^k = \frac{\partial}{\partial x} \sum_{k=0}^n {n \choose k} x^k = n(x+1)^{n-1} = n 2^{n-1} \end{align} \]
2. 求和\[ \sum_{k=0}^n k^2 {n \choose k} \]
   解法：\[ \begin{align} (x = 1 时)\sum_{k=0}^n k^2 {n \choose k} & = \sum_{k=0}^n {n \choose k}\frac{\partial}{\partial x} x \frac{\partial}{\partial x} x^k = \frac{\partial}{\partial x} x \frac{\partial}{\partial x} \sum_{k=0}^n {n \choose k} x^k = …… \end{align} \]