Calculate $lim_{x to 0} frac{ln(1+2x)}{x^2}$ with the help of l'Hospital's and Bernoullie's rule.












3












$begingroup$



Task:



Calculate $$lim_{x to 0} frac{ln(1+2x)}{x^2}$$ with the help of
l'Hospital's and Bernoullie's rule.




My thoughts:



Because $mathcal{D}(f)={xmid xinmathbb{R} land xneq 0}$ the function is undefined for $0$ and therefore, I need to find out, whether the function has a limit or only one-sided limits. In order to do that, I'll just calculate the one sided limits. If $$lim_{x to 0^+} frac{ln(1+2x)}{x^2}neq lim_{x to 0^-} frac{ln(1+2x)}{x^2} implies lim_{x to 0} frac{ln(1+2x)}{x^2} text{ doesn't exist}$$



$lim_{x to 0^-} frac{ln(1+2x)}{x^2}overbrace{=}^{l'Hospital}=lim_{x to 0^-} frac{2/(2x+1)}{2x}=lim_{x to 0^-}frac{1}{x(2x+1)}overbrace{=}^{product- rule}underbrace{lim_{x to 0^-}frac{1}{(2x+1)}}_{=1}cdot underbrace{lim_{x to 0^-}frac{1}{x}}_{(*)}=1cdot (*)=(*)$



$(*)$: If $x$ is small, than $1/x$ gets proportional bigger. Let $M>0$ and let $delta = 1/M$. Than $-1/x<frac{-1}{1/M}=-M;forall (-delta)<x<0$. Since $M$ can be arbitrarily large: $$lim_{x to 0^-} frac1x=-infty$$



$lim_{x to 0^-}$ analogue. $$lim_{x to 0^+} frac{ln(1+2x)}{x^2} = cdots = lim_{x to 0^+} frac1x=infty$$ $implies lim_{x to 0}$ doesn't exist.



Is this proof correct?










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    Yes the searched limit doesn't exist.
    $endgroup$
    – Dr. Sonnhard Graubner
    Dec 10 '18 at 20:16










  • $begingroup$
    Can you use Maclaurin series? Logarithm expansion immediately gives you the resut
    $endgroup$
    – Alex
    Dec 10 '18 at 20:23






  • 1




    $begingroup$
    @Alex I've never heard of the Maclaurin Series, but I'm curious how that proof would look like! Please share it.
    $endgroup$
    – Doesbaddel
    Dec 10 '18 at 20:25








  • 2




    $begingroup$
    Maybe you have heard of Taylor series? It’s practically the same thing. Actually I don’t know why they are called differently, anybody knows?
    $endgroup$
    – tommy1996q
    Dec 10 '18 at 20:30








  • 2




    $begingroup$
    @tommy1996q It's just nomenclature. A Maclaurin series is just a Taylor expansion about $x=0$.
    $endgroup$
    – bob.sacamento
    Dec 10 '18 at 20:35
















3












$begingroup$



Task:



Calculate $$lim_{x to 0} frac{ln(1+2x)}{x^2}$$ with the help of
l'Hospital's and Bernoullie's rule.




My thoughts:



Because $mathcal{D}(f)={xmid xinmathbb{R} land xneq 0}$ the function is undefined for $0$ and therefore, I need to find out, whether the function has a limit or only one-sided limits. In order to do that, I'll just calculate the one sided limits. If $$lim_{x to 0^+} frac{ln(1+2x)}{x^2}neq lim_{x to 0^-} frac{ln(1+2x)}{x^2} implies lim_{x to 0} frac{ln(1+2x)}{x^2} text{ doesn't exist}$$



$lim_{x to 0^-} frac{ln(1+2x)}{x^2}overbrace{=}^{l'Hospital}=lim_{x to 0^-} frac{2/(2x+1)}{2x}=lim_{x to 0^-}frac{1}{x(2x+1)}overbrace{=}^{product- rule}underbrace{lim_{x to 0^-}frac{1}{(2x+1)}}_{=1}cdot underbrace{lim_{x to 0^-}frac{1}{x}}_{(*)}=1cdot (*)=(*)$



$(*)$: If $x$ is small, than $1/x$ gets proportional bigger. Let $M>0$ and let $delta = 1/M$. Than $-1/x<frac{-1}{1/M}=-M;forall (-delta)<x<0$. Since $M$ can be arbitrarily large: $$lim_{x to 0^-} frac1x=-infty$$



$lim_{x to 0^-}$ analogue. $$lim_{x to 0^+} frac{ln(1+2x)}{x^2} = cdots = lim_{x to 0^+} frac1x=infty$$ $implies lim_{x to 0}$ doesn't exist.



Is this proof correct?










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    Yes the searched limit doesn't exist.
    $endgroup$
    – Dr. Sonnhard Graubner
    Dec 10 '18 at 20:16










  • $begingroup$
    Can you use Maclaurin series? Logarithm expansion immediately gives you the resut
    $endgroup$
    – Alex
    Dec 10 '18 at 20:23






  • 1




    $begingroup$
    @Alex I've never heard of the Maclaurin Series, but I'm curious how that proof would look like! Please share it.
    $endgroup$
    – Doesbaddel
    Dec 10 '18 at 20:25








  • 2




    $begingroup$
    Maybe you have heard of Taylor series? It’s practically the same thing. Actually I don’t know why they are called differently, anybody knows?
    $endgroup$
    – tommy1996q
    Dec 10 '18 at 20:30








  • 2




    $begingroup$
    @tommy1996q It's just nomenclature. A Maclaurin series is just a Taylor expansion about $x=0$.
    $endgroup$
    – bob.sacamento
    Dec 10 '18 at 20:35














3












3








3





$begingroup$



Task:



Calculate $$lim_{x to 0} frac{ln(1+2x)}{x^2}$$ with the help of
l'Hospital's and Bernoullie's rule.




My thoughts:



Because $mathcal{D}(f)={xmid xinmathbb{R} land xneq 0}$ the function is undefined for $0$ and therefore, I need to find out, whether the function has a limit or only one-sided limits. In order to do that, I'll just calculate the one sided limits. If $$lim_{x to 0^+} frac{ln(1+2x)}{x^2}neq lim_{x to 0^-} frac{ln(1+2x)}{x^2} implies lim_{x to 0} frac{ln(1+2x)}{x^2} text{ doesn't exist}$$



$lim_{x to 0^-} frac{ln(1+2x)}{x^2}overbrace{=}^{l'Hospital}=lim_{x to 0^-} frac{2/(2x+1)}{2x}=lim_{x to 0^-}frac{1}{x(2x+1)}overbrace{=}^{product- rule}underbrace{lim_{x to 0^-}frac{1}{(2x+1)}}_{=1}cdot underbrace{lim_{x to 0^-}frac{1}{x}}_{(*)}=1cdot (*)=(*)$



$(*)$: If $x$ is small, than $1/x$ gets proportional bigger. Let $M>0$ and let $delta = 1/M$. Than $-1/x<frac{-1}{1/M}=-M;forall (-delta)<x<0$. Since $M$ can be arbitrarily large: $$lim_{x to 0^-} frac1x=-infty$$



$lim_{x to 0^-}$ analogue. $$lim_{x to 0^+} frac{ln(1+2x)}{x^2} = cdots = lim_{x to 0^+} frac1x=infty$$ $implies lim_{x to 0}$ doesn't exist.



Is this proof correct?










share|cite|improve this question











$endgroup$





Task:



Calculate $$lim_{x to 0} frac{ln(1+2x)}{x^2}$$ with the help of
l'Hospital's and Bernoullie's rule.




My thoughts:



Because $mathcal{D}(f)={xmid xinmathbb{R} land xneq 0}$ the function is undefined for $0$ and therefore, I need to find out, whether the function has a limit or only one-sided limits. In order to do that, I'll just calculate the one sided limits. If $$lim_{x to 0^+} frac{ln(1+2x)}{x^2}neq lim_{x to 0^-} frac{ln(1+2x)}{x^2} implies lim_{x to 0} frac{ln(1+2x)}{x^2} text{ doesn't exist}$$



$lim_{x to 0^-} frac{ln(1+2x)}{x^2}overbrace{=}^{l'Hospital}=lim_{x to 0^-} frac{2/(2x+1)}{2x}=lim_{x to 0^-}frac{1}{x(2x+1)}overbrace{=}^{product- rule}underbrace{lim_{x to 0^-}frac{1}{(2x+1)}}_{=1}cdot underbrace{lim_{x to 0^-}frac{1}{x}}_{(*)}=1cdot (*)=(*)$



$(*)$: If $x$ is small, than $1/x$ gets proportional bigger. Let $M>0$ and let $delta = 1/M$. Than $-1/x<frac{-1}{1/M}=-M;forall (-delta)<x<0$. Since $M$ can be arbitrarily large: $$lim_{x to 0^-} frac1x=-infty$$



$lim_{x to 0^-}$ analogue. $$lim_{x to 0^+} frac{ln(1+2x)}{x^2} = cdots = lim_{x to 0^+} frac1x=infty$$ $implies lim_{x to 0}$ doesn't exist.



Is this proof correct?







real-analysis calculus proof-verification






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 10 '18 at 20:27







Doesbaddel

















asked Dec 10 '18 at 20:13









DoesbaddelDoesbaddel

34011




34011








  • 1




    $begingroup$
    Yes the searched limit doesn't exist.
    $endgroup$
    – Dr. Sonnhard Graubner
    Dec 10 '18 at 20:16










  • $begingroup$
    Can you use Maclaurin series? Logarithm expansion immediately gives you the resut
    $endgroup$
    – Alex
    Dec 10 '18 at 20:23






  • 1




    $begingroup$
    @Alex I've never heard of the Maclaurin Series, but I'm curious how that proof would look like! Please share it.
    $endgroup$
    – Doesbaddel
    Dec 10 '18 at 20:25








  • 2




    $begingroup$
    Maybe you have heard of Taylor series? It’s practically the same thing. Actually I don’t know why they are called differently, anybody knows?
    $endgroup$
    – tommy1996q
    Dec 10 '18 at 20:30








  • 2




    $begingroup$
    @tommy1996q It's just nomenclature. A Maclaurin series is just a Taylor expansion about $x=0$.
    $endgroup$
    – bob.sacamento
    Dec 10 '18 at 20:35














  • 1




    $begingroup$
    Yes the searched limit doesn't exist.
    $endgroup$
    – Dr. Sonnhard Graubner
    Dec 10 '18 at 20:16










  • $begingroup$
    Can you use Maclaurin series? Logarithm expansion immediately gives you the resut
    $endgroup$
    – Alex
    Dec 10 '18 at 20:23






  • 1




    $begingroup$
    @Alex I've never heard of the Maclaurin Series, but I'm curious how that proof would look like! Please share it.
    $endgroup$
    – Doesbaddel
    Dec 10 '18 at 20:25








  • 2




    $begingroup$
    Maybe you have heard of Taylor series? It’s practically the same thing. Actually I don’t know why they are called differently, anybody knows?
    $endgroup$
    – tommy1996q
    Dec 10 '18 at 20:30








  • 2




    $begingroup$
    @tommy1996q It's just nomenclature. A Maclaurin series is just a Taylor expansion about $x=0$.
    $endgroup$
    – bob.sacamento
    Dec 10 '18 at 20:35








1




1




$begingroup$
Yes the searched limit doesn't exist.
$endgroup$
– Dr. Sonnhard Graubner
Dec 10 '18 at 20:16




$begingroup$
Yes the searched limit doesn't exist.
$endgroup$
– Dr. Sonnhard Graubner
Dec 10 '18 at 20:16












$begingroup$
Can you use Maclaurin series? Logarithm expansion immediately gives you the resut
$endgroup$
– Alex
Dec 10 '18 at 20:23




$begingroup$
Can you use Maclaurin series? Logarithm expansion immediately gives you the resut
$endgroup$
– Alex
Dec 10 '18 at 20:23




1




1




$begingroup$
@Alex I've never heard of the Maclaurin Series, but I'm curious how that proof would look like! Please share it.
$endgroup$
– Doesbaddel
Dec 10 '18 at 20:25






$begingroup$
@Alex I've never heard of the Maclaurin Series, but I'm curious how that proof would look like! Please share it.
$endgroup$
– Doesbaddel
Dec 10 '18 at 20:25






2




2




$begingroup$
Maybe you have heard of Taylor series? It’s practically the same thing. Actually I don’t know why they are called differently, anybody knows?
$endgroup$
– tommy1996q
Dec 10 '18 at 20:30






$begingroup$
Maybe you have heard of Taylor series? It’s practically the same thing. Actually I don’t know why they are called differently, anybody knows?
$endgroup$
– tommy1996q
Dec 10 '18 at 20:30






2




2




$begingroup$
@tommy1996q It's just nomenclature. A Maclaurin series is just a Taylor expansion about $x=0$.
$endgroup$
– bob.sacamento
Dec 10 '18 at 20:35




$begingroup$
@tommy1996q It's just nomenclature. A Maclaurin series is just a Taylor expansion about $x=0$.
$endgroup$
– bob.sacamento
Dec 10 '18 at 20:35










2 Answers
2






active

oldest

votes


















2












$begingroup$

Yes your evaluation is fine, to check it by standard limits, we have that



$$ frac{ln(1+2x)}{x^2}=frac{ln(1+2x)}{2x}frac{2}{x}to 1cdotpminfty$$



therefore the limit doesn't exist.



For the proof of the standard limit refer to




  • Determine the following limit as x approaches 0: $frac{ln(1+x)}x$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Thank you very much, that shortens the proof significantly!
    $endgroup$
    – Doesbaddel
    Dec 11 '18 at 18:14






  • 1




    $begingroup$
    @Doesbaddel You are welcome! That's strictly related to some general suggestions given here
    $endgroup$
    – gimusi
    Dec 11 '18 at 18:15



















2












$begingroup$

Since as $x to 0 log (1 + 2x) to 0$, you can expand $log$ function around $x=0$ to get (first term is enough) $log (1+2x) sim 2x$, and the fraction becomes $frac{2}{x}$ that certainly diverges






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Thanks, way shorter than my thoughts.
    $endgroup$
    – Doesbaddel
    Dec 11 '18 at 18:13











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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









2












$begingroup$

Yes your evaluation is fine, to check it by standard limits, we have that



$$ frac{ln(1+2x)}{x^2}=frac{ln(1+2x)}{2x}frac{2}{x}to 1cdotpminfty$$



therefore the limit doesn't exist.



For the proof of the standard limit refer to




  • Determine the following limit as x approaches 0: $frac{ln(1+x)}x$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Thank you very much, that shortens the proof significantly!
    $endgroup$
    – Doesbaddel
    Dec 11 '18 at 18:14






  • 1




    $begingroup$
    @Doesbaddel You are welcome! That's strictly related to some general suggestions given here
    $endgroup$
    – gimusi
    Dec 11 '18 at 18:15
















2












$begingroup$

Yes your evaluation is fine, to check it by standard limits, we have that



$$ frac{ln(1+2x)}{x^2}=frac{ln(1+2x)}{2x}frac{2}{x}to 1cdotpminfty$$



therefore the limit doesn't exist.



For the proof of the standard limit refer to




  • Determine the following limit as x approaches 0: $frac{ln(1+x)}x$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Thank you very much, that shortens the proof significantly!
    $endgroup$
    – Doesbaddel
    Dec 11 '18 at 18:14






  • 1




    $begingroup$
    @Doesbaddel You are welcome! That's strictly related to some general suggestions given here
    $endgroup$
    – gimusi
    Dec 11 '18 at 18:15














2












2








2





$begingroup$

Yes your evaluation is fine, to check it by standard limits, we have that



$$ frac{ln(1+2x)}{x^2}=frac{ln(1+2x)}{2x}frac{2}{x}to 1cdotpminfty$$



therefore the limit doesn't exist.



For the proof of the standard limit refer to




  • Determine the following limit as x approaches 0: $frac{ln(1+x)}x$






share|cite|improve this answer









$endgroup$



Yes your evaluation is fine, to check it by standard limits, we have that



$$ frac{ln(1+2x)}{x^2}=frac{ln(1+2x)}{2x}frac{2}{x}to 1cdotpminfty$$



therefore the limit doesn't exist.



For the proof of the standard limit refer to




  • Determine the following limit as x approaches 0: $frac{ln(1+x)}x$







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Dec 10 '18 at 21:10









gimusigimusi

92.8k84494




92.8k84494












  • $begingroup$
    Thank you very much, that shortens the proof significantly!
    $endgroup$
    – Doesbaddel
    Dec 11 '18 at 18:14






  • 1




    $begingroup$
    @Doesbaddel You are welcome! That's strictly related to some general suggestions given here
    $endgroup$
    – gimusi
    Dec 11 '18 at 18:15


















  • $begingroup$
    Thank you very much, that shortens the proof significantly!
    $endgroup$
    – Doesbaddel
    Dec 11 '18 at 18:14






  • 1




    $begingroup$
    @Doesbaddel You are welcome! That's strictly related to some general suggestions given here
    $endgroup$
    – gimusi
    Dec 11 '18 at 18:15
















$begingroup$
Thank you very much, that shortens the proof significantly!
$endgroup$
– Doesbaddel
Dec 11 '18 at 18:14




$begingroup$
Thank you very much, that shortens the proof significantly!
$endgroup$
– Doesbaddel
Dec 11 '18 at 18:14




1




1




$begingroup$
@Doesbaddel You are welcome! That's strictly related to some general suggestions given here
$endgroup$
– gimusi
Dec 11 '18 at 18:15




$begingroup$
@Doesbaddel You are welcome! That's strictly related to some general suggestions given here
$endgroup$
– gimusi
Dec 11 '18 at 18:15











2












$begingroup$

Since as $x to 0 log (1 + 2x) to 0$, you can expand $log$ function around $x=0$ to get (first term is enough) $log (1+2x) sim 2x$, and the fraction becomes $frac{2}{x}$ that certainly diverges






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Thanks, way shorter than my thoughts.
    $endgroup$
    – Doesbaddel
    Dec 11 '18 at 18:13
















2












$begingroup$

Since as $x to 0 log (1 + 2x) to 0$, you can expand $log$ function around $x=0$ to get (first term is enough) $log (1+2x) sim 2x$, and the fraction becomes $frac{2}{x}$ that certainly diverges






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Thanks, way shorter than my thoughts.
    $endgroup$
    – Doesbaddel
    Dec 11 '18 at 18:13














2












2








2





$begingroup$

Since as $x to 0 log (1 + 2x) to 0$, you can expand $log$ function around $x=0$ to get (first term is enough) $log (1+2x) sim 2x$, and the fraction becomes $frac{2}{x}$ that certainly diverges






share|cite|improve this answer









$endgroup$



Since as $x to 0 log (1 + 2x) to 0$, you can expand $log$ function around $x=0$ to get (first term is enough) $log (1+2x) sim 2x$, and the fraction becomes $frac{2}{x}$ that certainly diverges







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Dec 10 '18 at 21:11









AlexAlex

14.3k42134




14.3k42134












  • $begingroup$
    Thanks, way shorter than my thoughts.
    $endgroup$
    – Doesbaddel
    Dec 11 '18 at 18:13


















  • $begingroup$
    Thanks, way shorter than my thoughts.
    $endgroup$
    – Doesbaddel
    Dec 11 '18 at 18:13
















$begingroup$
Thanks, way shorter than my thoughts.
$endgroup$
– Doesbaddel
Dec 11 '18 at 18:13




$begingroup$
Thanks, way shorter than my thoughts.
$endgroup$
– Doesbaddel
Dec 11 '18 at 18:13


















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