Calculate $lim_{x to 0} frac{ln(1+2x)}{x^2}$ with the help of l'Hospital's and Bernoullie's rule.
$begingroup$
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?
real-analysis calculus proof-verification
$endgroup$
|
show 3 more comments
$begingroup$
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?
real-analysis calculus proof-verification
$endgroup$
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
|
show 3 more comments
$begingroup$
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?
real-analysis calculus proof-verification
$endgroup$
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
real-analysis calculus proof-verification
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
|
show 3 more comments
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
|
show 3 more comments
2 Answers
2
active
oldest
votes
$begingroup$
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: $frac{ln(1+x)}x$
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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
$endgroup$
$begingroup$
Thanks, way shorter than my thoughts.
$endgroup$
– Doesbaddel
Dec 11 '18 at 18:13
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
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$begingroup$
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: $frac{ln(1+x)}x$
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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: $frac{ln(1+x)}x$
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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: $frac{ln(1+x)}x$
$endgroup$
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: $frac{ln(1+x)}x$
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
add a comment |
$begingroup$
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
add a comment |
$begingroup$
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
$endgroup$
$begingroup$
Thanks, way shorter than my thoughts.
$endgroup$
– Doesbaddel
Dec 11 '18 at 18:13
add a comment |
$begingroup$
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
$endgroup$
$begingroup$
Thanks, way shorter than my thoughts.
$endgroup$
– Doesbaddel
Dec 11 '18 at 18:13
add a comment |
$begingroup$
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
$endgroup$
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
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
add a comment |
$begingroup$
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
add a comment |
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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