a_n \left( {x + a} \right)^n = a_n \sum\limits_{j = 0}^n {\left( \begin{array}{c}
 n \\ 
 j \\ 
 \end{array} \right)a^{n - j} x^j } = a_n x^n + \sum\limits_{j = 0}^{n - 1} {\left( \begin{array}{c}
 n \\ 
 j \\ 
 \end{array} \right)a^{n - j} x^j }