If I have 

$$
\sum\_{n=1}^K nx^{n-1} = \frac{d}{dx}\frac{1-x^{K+1}}{1-x}.
$$

How to calculate this formula based on above equation ?

$$a = \sum\_{n=2}^{K}n\left(\frac{\gamma}{2}\right)^{n-1}\rho^n$$