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BTW: the code is

[TEX]\begin{gathered} \sum\limits_{n = 0}^\infty {x^n } = \frac{1}{{1 - x}} \hfill \\ \sum\limits_{n = 0}^\infty {nx^{n - 1} } = \frac{1}{{\left( {1 - x} \right)^2 }} \hfill \\ \sum\limits_{n = 0}^\infty {nx^n } = \frac{x}{{\left( {1 - x} \right)^2 }} \hfill \\ \end{gathered}[/TEX]