I think I should know this already, but I don't; yet I need it, so would you please help me sum this...(adsbygoogle = window.adsbygoogle || []).push({});

[tex]\sum_{n=k}^{\infty} \left(\begin{array}{c}n\\k\end{array}\right) \left( \frac{-z}{1-z}\right) ^{n} = (1-z)(-z)^{k}[/tex]

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# Homework Help: This ought to be an easy sum

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