First Derivative of PGF of Negative Binomial Distribution/First Form

Theorem
Let $X$ be a discrete random variable with the negative binomial distribution (first form) with parameters $n$ and $p$.

Then the first derivative of the PGF of $X$ w.r.t. $s$ is:


 * $\dfrac {\mathrm d} {\mathrm d s} \Pi_X \left({s}\right) = \dfrac {n p} q \left({\dfrac q {1 - ps} }\right)^{n+1}$

Proof
The Probability Generating Function of Negative Binomial Distribution (First Form) is:


 * $\Pi_X \left({s}\right) = \left({\dfrac q {1 - ps}}\right)^n$

We have that for a given negative binomial distribution, $n, p$ and $q$ are constant.

Thus we have:

Hence the result.