2 = \sum\limits_{i = 1}^3 {\left( \begin{array}{c}
 x_i \\ 
 i \\ 
 \end{array} \right)} \ge \sum\limits_{i = 1}^3 {\left( \begin{array}{c}
 i \\ 
 i \\ 
 \end{array} \right)} = 3