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L'Hospital's rule question.

  • Thread starter 1337caesar
  • Start date
so i've been working on this for a while now and i just cant get it.

Evaluate the limit using L'Hospital's rule if necessary

lim X -> INF
(x/x+1) ^7x

i know i need to do...


exp(lim x-> inf 7x ln(x/x+1))

from there i'm stuck.

i appreciate any help
 

Answers and Replies

275
2
You could break it up into numerator and denominator, f(x)/g(x) lets say, where f(x) = x^7x and g(x) = (x+1)^7x l'hopital's rule says that if both f(x) and g(x) are divergent, then x--> inf f(x)/g(x) = x-->inf f'(x) / g'(x) , so you take the derivative of the top and bottom separately... you're doing to have to do this until its not divergent anymore... or 8 times? i guess?
i see what you're going for with the exp(7xln...) but i think thats probably harder / more work. Good luck
 
Last edited:
Dick
Science Advisor
Homework Helper
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7x*ln(x/(x+1)) is a 0*infinity limit. Use l'Hopital again. Write it as 7*ln(x/(x+1))/(1/x). You can make life a little easier by changing ln(x/(x+1)) to -ln((x+1)/x) and simplifying inside the log.
 
Dick
Science Advisor
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well, we can see that the answer is going to be one, because in the limit that x --> inf, x = x+1 therefore x/x+1 = 1, so 1^anything even inf is still 1.
Now to prove that more rigorously, you break it up into numerator and denominator, f(x)/g(x) lets say, where f(x) = x^7x and g(x) = (x+1)^7x l'hopital's rule says that if both f(x) and g(x) are divergent, then x--> inf f(x)/g(x) = x-->inf f'(x) / g'(x) , so you take the derivative of the top and bottom separately... you're doing to have to do this until its not divergent anymore... or 8 times? i guess?
i see what you're going for with the exp(7xln...) but i think thats probably harder / more work. Good luck
The limit isn't 1. And taking the log does make it easier.
 
exk
119
0
As Dick said taking the log makes it a lot easier.

You may have to apply l'Hopital more than once.
 
thanks guys i got it by taking ln of the whole thing and l'hopitaling the thing twice. ended up being e^(-7)
 

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