COEFFICIENT of \[{{x}^{N-R}}{{a}^{r}}\]and \[{{x}^{r}}{{a}^{n-r}}\]is = \[\FRAC{^{n}{{C}_{r}}}{^{n}{{C}_{n-r}}}=\frac{^{n}{{C}_{r}}}{^{n}{{C}_{r}}}=\frac{1}{1}\]