<html><head><meta name="color-scheme" content="light dark"></head><body><pre style="word-wrap: break-word; white-space: pre-wrap;">Sečtěte řadu
$$
\sum _{n=1}^{\infty }n\left (n+2\right ){x}^{n}=3\,x\left ({\frac 
{1-x}{\left (x-1\right )^{4}x}}-{\frac {1-x}{3\,\left (x-1\right )
^{4}{x}^{2}}}+{\frac {1}{3\,{x}^{2}}}\right ) \quad .
$$
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