Binomial Theorem

If b is very small as compared to the value of a, so that the cube and other higher powers of \({b \over a}\) can be neglected in the identity \({1 \over {a - b}} + {1 \over {a - 2b}} + {1 \over {a - 3b}} + ..... + {1 \over {a - nb}} = \alpha n + \beta {n^2} + \gamma {n^3}\), then the value of \(\gamma\) is :