Изменения

<center><math>

<tex>\dfrac{1}{1+z}=\sum_{n=0}^\infty (-1)^n z^n </tex>

<tex>\dfrac{7}{(1-z)^2}=\sum_{n=0}^\infty 7(n+1) z^n </tex>

<tex>\dfrac{z}{(1-z)^2}=\sum_{n=0}^\infty n z^n .</tex>

</center>