Sum of Geometric Sequence/Corollary 2

Theorem
Let $x$ be an element of one of the standard number fields: $\Q, \R, \C$ such that $x \ne 1$.

Let $n \in \N_{>0}$.

Then:
 * $\displaystyle \sum_{j \mathop = 0}^{n - 1} j x^j = \frac {\left({n - 1}\right) x^{n + 1} - n x^n + x} {\left({x - 1} \right)^2}$