Spoètìte souèet mocninné øady
$$
\sum _{n=1}^{\infty }\left (-1\right )^{n+1}{n}^{2}{x}^{n}=x\left 
(\left (x+1\right )^{-2}-{\frac {2\,x}{\left (x+1\right )^{3}}}
\right ) \quad .
$$
