proof of radius of convergence
According to Cauchy’s root test a power series
is absolutely convergent if
This is obviously true if
In the same way we see that the series is divergent if
which means that the right hand side is the radius of convergence of the power series.Now from the ratio test
we see that the power series is absolutely convergent if
Again this is true if
The series is divergent if
as follows from the ratio test in the same way. So we see that in this way too we can the radius of convergence.