Using L'Hospital's rule with roots and log functions

In summary: Looks like the conversation is about using L'Hopital's rule to determine the limit of x^(-1/2)ln(x) as x approaches 0. However, it is pointed out that L'Hopital's rule can only be applied to a limit of a quotient, and in this case, it is not a quotient. It is then suggested to rewrite the expression as ln(x)/x^(-1/2) to make it a quotient and thus applicable for L'Hopital's rule. However, there is some confusion about rewriting x^(1/2) as x^(-1/2), and it is clarified that while it cannot be rewritten in that way, it can be written as a fraction with a negative
  • #1
fiziksfun
78
0
Can someone help me use L'HOP to determine

lim x -> 0 [ [tex]\sqrt{x}[/tex]*ln(x) ]

? I'm confused!
 
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  • #2


fiziksfun said:
Can someone help me use L'HOP to determine

lim x -> 0 [ [tex]\sqrt{x}[/tex]*ln(x) ]

? I'm confused!
L'Hopital's rule doesn't apply here. One can only apply L'Hopital's rule for a limit of a quotient and only then when the limit is undefined.
 
  • #3


Write it as ln(x)/x^(-1/2). Now it's a quotient and infinity/infinity. Looks good for l'hop.
 
  • #4


Dick said:
Write it as ln(x)/x^(-1/2). Now it's a quotient and infinity/infinity. Looks good for l'hop.
*Hangs head in shame and shuffles back into the Physics section*
 
  • #5


Dick said:
Write it as ln(x)/x^(-1/2). Now it's a quotient and infinity/infinity. Looks good for l'hop.

why can i rewrite x^(1/2) as x^(-1/2) ? I don't understand.
 
  • #6


fiziksfun said:
why can i rewrite x^(1/2) as x^(-1/2) ? I don't understand.

You can't, but you can write [tex]x^{1/2}[/tex] as [tex]\frac{1}{x^{-1/2}}[/tex], which is what Dick has done above.
 
  • #7


fiziksfun said:
why can i rewrite x^(1/2) as x^(-1/2) ? I don't understand.
You can't rewrite x^(1/2) as x^(-1/2), but you can rewrite it as,

[tex]x^{1/2} = \frac{1}{x^{-1/2}}[/tex]

as Dick suggests.

Edit: Get out of my head cristo :-p
 
  • #8


Hootenanny said:
Edit: Get out of my head cristo :-p

:wink:
 

FAQ: Using L'Hospital's rule with roots and log functions

What is L'Hospital's rule and when is it used?

L'Hospital's rule is a mathematical tool used to evaluate limits involving indeterminate forms, such as 0/0 or ∞/∞. It states that if the limit of a function f(x) over g(x) is an indeterminate form, then the limit of the derivative of f(x) over the derivative of g(x) is equivalent to the original limit. It is typically used when other methods, such as direct substitution, are unsuccessful in evaluating a limit.

How is L'Hospital's rule applied to functions with roots?

When applying L'Hospital's rule to a function with a root, the function must be rewritten in a way that allows for the use of derivatives. This can be done by rewriting the root as a power, then taking the natural logarithm of both sides. After taking the derivative, the limit can be evaluated as usual.

Can L'Hospital's rule be used with logarithmic functions?

Yes, L'Hospital's rule can be applied to logarithmic functions. The function must be rewritten in a way that allows for the use of derivatives, such as using the properties of logarithms to rewrite the function as a quotient. After taking the derivative, the limit can be evaluated as usual.

What are some common mistakes when using L'Hospital's rule with roots and log functions?

Some common mistakes when using L'Hospital's rule with roots and log functions include not rewriting the function in a way that allows for the use of derivatives, not taking the derivative correctly, or not checking for other possible methods of evaluating the limit first. It is important to carefully follow the steps of L'Hospital's rule and double check the final result.

Are there any restrictions to using L'Hospital's rule with roots and log functions?

Yes, there are some restrictions to using L'Hospital's rule with roots and log functions. The function must be in an indeterminate form and the limit must exist at the point of evaluation. In addition, the functions must be differentiable at the point of evaluation, and the derivative of the denominator must not approach 0 at the point of evaluation. If these restrictions are not met, L'Hospital's rule cannot be used.

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