is there a closed-form for the following power series?

$\displaystyle \sum_{i = a}^{\infty} i \cdot x^{n \cdot i}$

p.s. i just know that

1. $\displaystyle \sum_{i = 0}^{\infty} x^{n \cdot i} = \frac{1}{1- x^n}$

2. $\displaystyle \sum_{i = 0}^{\infty} i \cdot x^{i} = \frac{x}{(1-x)^2}$

Thanks