# Math Help - Question about Series convergance

1. ## Question about Series convergance

My book has writen this. Series is attached

How he has converged the series

2. That's almost a geometric series. There is a general formula that
$\sum_{n=0}^\infty ar^n= \frac{a}{1- r}$.

Now, the derivative, term by term, is $\sum_{n=0}^\infty a_n r^{n-1}$ and that must be equal to the derivative, with respect to r, of $\frac{a}{1- r}$.

3. Why book has used (Square) in denominator???