009C Sample Final 2, Problem 4

(a) Find the radius of convergence for the power series

$\sum _{n=1}^{+\infty }(-1)^{n}{\frac {x^{n}}{n}}.$

(b) Find the interval of convergence of the above series.

Foundations:
Ratio Test
Let  $\sum a_{n}$  be a series and  $L=\lim _{n\rightarrow \infty }{\bigg |}{\frac {a_{n+1}}{a_{n}}}{\bigg |}.$
Then,
If  $L<1,$  the series is absolutely convergent.
If  $L>1,$  the series is divergent.
If  $L=1,$  the test is inconclusive.

Solution:

(a)

Step 1:

Step 2:

(b)

Step 1:

Step 2:

Final Answer:
(a)
(b)

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