L'Hopital's Rule/Limit troubles

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In summary, the speaker is having trouble with two math problems involving limits. They are following the rule of finding the derivative and then taking the limit, but are still getting incorrect answers. They are seeking advice for why this may be happening.
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Hello,
Well my math class finally started integrals after finishing H's Rule. But while I was doing my HW, I wouldn't get the correct answer but was sure I was following the rule. The rule is simple saying that if taking the limit gives indeteminate form ,then find dervative and then take limit. So anyway here are the two questions that I can't get
1) lim x->0+ (Ln[sin[x]])/(Ln[sin[2x]])
I took dervative and got Cot[x]/2cot[2x]. but since sin 0=0 i continued to get un defined so i took dervative again but got sin in the answer so i knew i was having some problem. The answer should be one but i can't seem to get it

2) Lim x->0 (xcos[x]+e^-x)/x^2
i did the dervative twice until the denominater was a constant and got the answer 0 but that isn't in domain. the correct answer is infinite but I again don't know how the textbook got that

Any ideas why I can't get the right answer =/
 
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  • #2
As for 2), check the limit of your original numerator as x goes to 0.

As for 1), understand why we have:
[tex]\frac{\cot(x)}{2\cot(2x)}=\frac{\sin(2x)}{2\sin(x)}\frac{\cos{x}}{\cos(2x)}=\frac{\cos^{2}(x)}{\cos(2x)}[/tex]
 

What is L'Hopital's Rule?

L'Hopital's Rule is a mathematical rule that allows us to evaluate limits involving indeterminate forms, such as 0/0 or ∞/∞. It states that if the limit of the quotient of two functions is indeterminate, then the limit of the quotient of their derivatives will be the same.

When should L'Hopital's Rule be used?

L'Hopital's Rule should only be used when the limit of a function is in an indeterminate form. This means that both the numerator and denominator of the function approach either 0 or ∞ as x approaches a certain value.

What are the common mistakes made when applying L'Hopital's Rule?

Some common mistakes when applying L'Hopital's Rule include not first verifying that the limit is in an indeterminate form, taking the derivative of only one part of the function, and not simplifying the resulting expression after applying the rule.

Is L'Hopital's Rule always applicable?

No, L'Hopital's Rule is not always applicable. It can only be used when the limit is in an indeterminate form. If the limit is not in an indeterminate form, then other methods, such as algebraic manipulation or substitution, must be used to evaluate the limit.

Are there any alternative methods to L'Hopital's Rule for evaluating limits?

Yes, there are alternative methods for evaluating limits, such as substitution, algebraic manipulation, and the squeeze theorem. It is important to consider all available methods and choose the most appropriate one for each specific limit problem.

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