Help with proving/disproving statements - series

**Stormey** · December 8th 2012, 01:46 AM

Hi guys,
i need to prove or disprove these statements:

a. if the series $\sum a_{n}$ is convergent, then $\sum \frac{1}{a_{n}}$ is divergent.

a. if the positive series $\sum a_{n}$ is convergent, then $\sum a_{n}^2$ is convergent.

if the statement is false, i need an example of contradiction.

basically i can really use some tips about how to approach these questions;
what tests should i use to determine if a statment is true or false (or maybe the first step should be merely my intuition), and in what ways can i prove it?

thanks in advanced!

**HallsofIvy** · December 8th 2012, 04:59 AM

What do you know? Do you know some examples of convergent and divergent series? Do you know "If $\sum a_n$ converges then $lim a_n= 0$ "? That should help greatly with the first one. Do you know the "ratio test"? That should help with the second. But be careful, it is precisely the condition under which the ratio test does not apply that is important!

**Stormey** · December 8th 2012, 06:43 AM

Hi HallsofIvy,

yes, i know that if $\sum a_n$ converges then $lim a_n= 0$ and about the "ratio test", for some unknown reason i didn't think to use it...
thanks a lot!