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Mathematics
OpenStudy (christos):

Limit, http://screencast.com/t/yFEkr1wbeM

OpenStudy (anonymous):

So your question is how he can multiply infinite by infinite.... right?

OpenStudy (anonymous):

consider infinite to be a some very large number.and if u multiply dis large no. wid another large number...then i will also be much much greater dat we cant detect.

OpenStudy (christos):

That's right, my question is how he can claim a result of inf * inf when that is undefined

OpenStudy (christos):

inf * inf is like something/0 *UNDEFINED*

OpenStudy (anonymous):

Well same as what madrockz said but I think that he can do it but it just means that the end # you get will be undefined with it

OpenStudy (christos):

I really doubt this to be the case but thanks alot for the input ! Any further opinion would be welcome !

OpenStudy (christos):

@hartnn Maybe you have any ideas ?

OpenStudy (anonymous):

Wait, what grade are you in...?

OpenStudy (christos):

university.

OpenStudy (anonymous):

That would explain why I dont know it. Lol

hartnn (hartnn):

i think the solution is incorrect x^(-1/2) = 0 when x->infinity so, denominator = 0*infinity = indeterminate

hartnn (hartnn):

and we need one more iteration of L h rule 2x^(1/2) / ln x ----> inf/inf form

hartnn (hartnn):

= 2 *(1/2) x^(-1/2) times x = x^(1/2) and x^(1/2) when x->infinity = infinity = final answer

OpenStudy (christos):

hm hold on

hartnn (hartnn):

eh, but wolf shows final answer = 0 i really can't make out where i made error....

OpenStudy (christos):

there is a tricky chain rule of ln(ln(x) that I think we missed there

OpenStudy (christos):

or no ?

hartnn (hartnn):

d/dx (ln ln x) = 1/ (x ln x)

OpenStudy (christos):

I go sort(x) ----> inf as well :O

hartnn (hartnn):

oh wait! sorry, that was x^(1/2) and not x^(-1/2)

OpenStudy (christos):

but if you differenciate it it will be minus

hartnn (hartnn):

why would u diff it ?

hartnn (hartnn):

as told earlier, in limits we consider inf*inf = inf

OpenStudy (christos):

soo that's not undefined ? It actually is inf * inf = inf ?

hartnn (hartnn):

only when x-> infinity without limits , its undefined

OpenStudy (christos):

inf * - inf = - inf ?

hartnn (hartnn):

seems madrocks was correct from the beginning... yes

OpenStudy (christos):

but wait, how can I come to the final step in the first place ?

hartnn (hartnn):

what ?

OpenStudy (christos):

nothing nothinng I solved it ! The only thing that confused me was the inf * inf part, thanks !

OpenStudy (christos):

I would like to ask some additional questions if possible, what if the limit is ----> -inf ? Would I operate the same way ? Are there any other undefined functions that would be "defined" under a specific situation in limits ? @hartnn

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