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-   -   Help with understanding of L'Hospitals Rule (http://www.physicsforums.com/showthread.php?t=591388)

 shocklightnin Mar29-12 01:27 AM

Help with understanding of L'Hospitals Rule

1. The problem statement, all variables and given/known data
This was a question from our lecture notes, just not sure how the prof arrived at the answer.

lim x->infinity (lnx)^2/x

2. Relevant equations

lim x->infinity (lnx)^2/x
lim x->infinity 2lnx/x

3. The attempt at a solution

so both the numerator and denominator are going towards infinity, and by L'H it the lim x->infinity 2lnx/x
so this means that the numerator is 'growing' faster than the denominator, a constant x? also, how does one arrive at the conclusion that the limit is 0?

 tazzzdo Mar29-12 01:57 AM

Re: Help with understanding of L'Hospitals Rule

(lnx)^2/x = (2/x)(ln x)

derivative of ln x = 1/x

Go from there. I'm drunk.

 RoshanBBQ Mar29-12 02:20 AM

Re: Help with understanding of L'Hospitals Rule

Quote:
 Quote by shocklightnin (Post 3839528) 1. The problem statement, all variables and given/known data This was a question from our lecture notes, just not sure how the prof arrived at the answer. lim x->infinity (lnx)^2/x 2. Relevant equations lim x->infinity (lnx)^2/x lim x->infinity 2lnx/x 3. The attempt at a solution so both the numerator and denominator are going towards infinity, and by L'H it the lim x->infinity 2lnx/x so this means that the numerator is 'growing' faster than the denominator, a constant x? also, how does one arrive at the conclusion that the limit is 0?
Just apply L'Hospital's rule again since you are still in an indeterminate inf/inf:
$$\frac{2}{x}$$

It should now be pretty sensible that it approaches 0 as x approaches infinity.

 shocklightnin Mar29-12 03:27 PM

Re: Help with understanding of L'Hospitals Rule

RoshanBBQ, thanks! Completely slipped my mind that sometimes we have to apply L'H more than once.

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