Determining the convergence or divergence with the given nth term

Nano-Passion
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I want to know why this particular approach is wrong so I can learn from my mistakes.

Homework Statement


a_n = \frac{ln(n^3)}{2n}

The Attempt at a Solution


For the sake of being time efficient, I will skip writing things like the limit as n approaches infinity etc.

a_n = \frac{ln(n^3)}{2n}
I let n^3 = u
a_n = \frac{ln(u)}{2n}
Using L'hopital's rule,
\frac{3n^2}{2n^3}
Applying it once more,
\frac{6n}{6n^2}
Which simplifys to
\frac{6}{12n}

This shows that the function diverges. Which is the wrong answer.
 
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I'm confused, \frac{6}{12n} converges to zero as n goes to infinity. Why do you interpret that as saying the function diverges?
 
Poopsilon said:
I'm confused, \frac{6}{12n} converges to zero as n goes to infinity. Why do you interpret that as saying the function diverges?

I'm shocked at my carelessness. I know that a number divided by infinity will equal 0. It is one of basic properties of infinite limits that I always use. So then how could I have done this careless mistake? It baffles me.
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...

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