Limlit of $frac{x-xln(1+x)-ln(1+x)}{x^3+x^2}$ at $0$ without l'Hôpital or Taylor series
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I have $f : x mapsto frac{ln(1+x)}{x}$ which derivative is $$frac{1}{x(1+x)}-frac{ln(1+x)}{x^2}$$
I want to find the limit as $x$ goes to $0$ of this derivative. I've tried simplifying the expression to $$frac{x - xln(1+x) - ln(1+x)}{x^3 + x^2}$$ but I'm still struggling. Also I can't use l'Hôpital or Taylor series. Any help would be appreciated, thanks in advance! (And sorry if I did anything wrong here)
logarithms limits-without-lhopital indeterminate-forms
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add a comment |
$begingroup$
I have $f : x mapsto frac{ln(1+x)}{x}$ which derivative is $$frac{1}{x(1+x)}-frac{ln(1+x)}{x^2}$$
I want to find the limit as $x$ goes to $0$ of this derivative. I've tried simplifying the expression to $$frac{x - xln(1+x) - ln(1+x)}{x^3 + x^2}$$ but I'm still struggling. Also I can't use l'Hôpital or Taylor series. Any help would be appreciated, thanks in advance! (And sorry if I did anything wrong here)
logarithms limits-without-lhopital indeterminate-forms
$endgroup$
add a comment |
$begingroup$
I have $f : x mapsto frac{ln(1+x)}{x}$ which derivative is $$frac{1}{x(1+x)}-frac{ln(1+x)}{x^2}$$
I want to find the limit as $x$ goes to $0$ of this derivative. I've tried simplifying the expression to $$frac{x - xln(1+x) - ln(1+x)}{x^3 + x^2}$$ but I'm still struggling. Also I can't use l'Hôpital or Taylor series. Any help would be appreciated, thanks in advance! (And sorry if I did anything wrong here)
logarithms limits-without-lhopital indeterminate-forms
$endgroup$
I have $f : x mapsto frac{ln(1+x)}{x}$ which derivative is $$frac{1}{x(1+x)}-frac{ln(1+x)}{x^2}$$
I want to find the limit as $x$ goes to $0$ of this derivative. I've tried simplifying the expression to $$frac{x - xln(1+x) - ln(1+x)}{x^3 + x^2}$$ but I'm still struggling. Also I can't use l'Hôpital or Taylor series. Any help would be appreciated, thanks in advance! (And sorry if I did anything wrong here)
logarithms limits-without-lhopital indeterminate-forms
logarithms limits-without-lhopital indeterminate-forms
edited Jan 5 at 0:58
mrtaurho
6,11271641
6,11271641
asked Jan 4 at 23:41
HeryonHeryon
214
214
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4 Answers
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First, you can decompose the first term in your derivative as:
$frac{x}{1+x} = frac{1}{x}-frac{1}{1+x}$.
Second, you can expand the second term using Taylor series:
$frac{ln(1+x)}{x^2}$
$ = frac{1}{x^2} left( x - frac{x^2}{2} + frac{x^3}{3} - .. right)$
$ = frac{1}{x} - frac{1}{2} + frac{x}{3} - $
Putting it together, your derivative is:
$frac{1}{x}-frac{1}{1+x} - frac{1}{x} + frac{1}{2} - frac{x}{3} +...$
- The $1/x$ terms cancel out.
$lim_{x rightarrow 0} frac{1}{1+x} = 1$.
$lim_{x rightarrow 0}$ of all terms with $x$ or higher powers of $x$ in the numerator is 0.
So you are left with $-1 + frac{1}{2} = -frac{1}{2}$.
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Clever, thanks. Any way to do this without Taylor series? (Just to know)
$endgroup$
– Heryon
Jan 5 at 0:06
add a comment |
$begingroup$
Your original function $f(x)=frac{ln(1+x)}{x}$ can be written using the taylor series expansion of $ln(1+x)$ as follows:
$$ln(1+x)=sum_{k=1}^{infty}frac{(-1)^{k+1}x^{k}}{k}=x-frac{x^2}{2}+frac{x^3}{3}-...$$
$$thereforefrac{ln(1+x)}{x}=sum_{k=1}^{infty}frac{(-1)^{k+1}x^{k-1}}{k}=sum_{k=0}^{infty}frac{(-1)^{k}x^{k}}{k+1}=1-frac{x}{2}+frac{x^2}{3}-...$$
$$frac{d}{dx}Bigg(frac{ln(1+x)}{x}Bigg)=frac{d}{dx}bigg(1-frac{x}{2}+frac{x^2}{3}-...bigg)=-frac{1}{2}+frac{2x}{3}-frac{3x^2}{4}+...$$
So,
$$lim_{xto0}Bigg(frac{d}{dx}Bigg(frac{ln(1+x)}{x}Bigg)Bigg)=lim_{xto0}bigg(-frac{1}{2}+frac{2x}{3}-frac{3x^2}{4}+...bigg)=fbox{$-frac{1}{2}$}$$
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$frac {1}{x(x+1)} - frac {ln(x+1)}{x^2} = frac {d}{dx} frac {ln(x+1)}{x}$
At which point I am inclined to use a series
$frac {d}{dx} (1 - frac x2 + cdots)$
as $x$ goes to $0$ equals $-frac 12$
Without using a series, I have.
$frac {d}{dx} frac {ln(x+1)}{x}$ evaluated at $x = 0$
is $lim_limits{hto 0} frac{ ln(1+h) - 1}{h^2}$
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Yes I can't use that too, edited the post, but thanks!
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– Heryon
Jan 5 at 0:33
add a comment |
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Because $frac{x}{1+x}<ln(1+x)<x$
So when $ x rightarrow 0+, f(x) rightarrow 1$
When $xrightarrow0-$ is similar
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add a comment |
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4 Answers
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4 Answers
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$begingroup$
First, you can decompose the first term in your derivative as:
$frac{x}{1+x} = frac{1}{x}-frac{1}{1+x}$.
Second, you can expand the second term using Taylor series:
$frac{ln(1+x)}{x^2}$
$ = frac{1}{x^2} left( x - frac{x^2}{2} + frac{x^3}{3} - .. right)$
$ = frac{1}{x} - frac{1}{2} + frac{x}{3} - $
Putting it together, your derivative is:
$frac{1}{x}-frac{1}{1+x} - frac{1}{x} + frac{1}{2} - frac{x}{3} +...$
- The $1/x$ terms cancel out.
$lim_{x rightarrow 0} frac{1}{1+x} = 1$.
$lim_{x rightarrow 0}$ of all terms with $x$ or higher powers of $x$ in the numerator is 0.
So you are left with $-1 + frac{1}{2} = -frac{1}{2}$.
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$begingroup$
Clever, thanks. Any way to do this without Taylor series? (Just to know)
$endgroup$
– Heryon
Jan 5 at 0:06
add a comment |
$begingroup$
First, you can decompose the first term in your derivative as:
$frac{x}{1+x} = frac{1}{x}-frac{1}{1+x}$.
Second, you can expand the second term using Taylor series:
$frac{ln(1+x)}{x^2}$
$ = frac{1}{x^2} left( x - frac{x^2}{2} + frac{x^3}{3} - .. right)$
$ = frac{1}{x} - frac{1}{2} + frac{x}{3} - $
Putting it together, your derivative is:
$frac{1}{x}-frac{1}{1+x} - frac{1}{x} + frac{1}{2} - frac{x}{3} +...$
- The $1/x$ terms cancel out.
$lim_{x rightarrow 0} frac{1}{1+x} = 1$.
$lim_{x rightarrow 0}$ of all terms with $x$ or higher powers of $x$ in the numerator is 0.
So you are left with $-1 + frac{1}{2} = -frac{1}{2}$.
$endgroup$
$begingroup$
Clever, thanks. Any way to do this without Taylor series? (Just to know)
$endgroup$
– Heryon
Jan 5 at 0:06
add a comment |
$begingroup$
First, you can decompose the first term in your derivative as:
$frac{x}{1+x} = frac{1}{x}-frac{1}{1+x}$.
Second, you can expand the second term using Taylor series:
$frac{ln(1+x)}{x^2}$
$ = frac{1}{x^2} left( x - frac{x^2}{2} + frac{x^3}{3} - .. right)$
$ = frac{1}{x} - frac{1}{2} + frac{x}{3} - $
Putting it together, your derivative is:
$frac{1}{x}-frac{1}{1+x} - frac{1}{x} + frac{1}{2} - frac{x}{3} +...$
- The $1/x$ terms cancel out.
$lim_{x rightarrow 0} frac{1}{1+x} = 1$.
$lim_{x rightarrow 0}$ of all terms with $x$ or higher powers of $x$ in the numerator is 0.
So you are left with $-1 + frac{1}{2} = -frac{1}{2}$.
$endgroup$
First, you can decompose the first term in your derivative as:
$frac{x}{1+x} = frac{1}{x}-frac{1}{1+x}$.
Second, you can expand the second term using Taylor series:
$frac{ln(1+x)}{x^2}$
$ = frac{1}{x^2} left( x - frac{x^2}{2} + frac{x^3}{3} - .. right)$
$ = frac{1}{x} - frac{1}{2} + frac{x}{3} - $
Putting it together, your derivative is:
$frac{1}{x}-frac{1}{1+x} - frac{1}{x} + frac{1}{2} - frac{x}{3} +...$
- The $1/x$ terms cancel out.
$lim_{x rightarrow 0} frac{1}{1+x} = 1$.
$lim_{x rightarrow 0}$ of all terms with $x$ or higher powers of $x$ in the numerator is 0.
So you are left with $-1 + frac{1}{2} = -frac{1}{2}$.
answered Jan 4 at 23:51
Aditya DuaAditya Dua
1,15418
1,15418
$begingroup$
Clever, thanks. Any way to do this without Taylor series? (Just to know)
$endgroup$
– Heryon
Jan 5 at 0:06
add a comment |
$begingroup$
Clever, thanks. Any way to do this without Taylor series? (Just to know)
$endgroup$
– Heryon
Jan 5 at 0:06
$begingroup$
Clever, thanks. Any way to do this without Taylor series? (Just to know)
$endgroup$
– Heryon
Jan 5 at 0:06
$begingroup$
Clever, thanks. Any way to do this without Taylor series? (Just to know)
$endgroup$
– Heryon
Jan 5 at 0:06
add a comment |
$begingroup$
Your original function $f(x)=frac{ln(1+x)}{x}$ can be written using the taylor series expansion of $ln(1+x)$ as follows:
$$ln(1+x)=sum_{k=1}^{infty}frac{(-1)^{k+1}x^{k}}{k}=x-frac{x^2}{2}+frac{x^3}{3}-...$$
$$thereforefrac{ln(1+x)}{x}=sum_{k=1}^{infty}frac{(-1)^{k+1}x^{k-1}}{k}=sum_{k=0}^{infty}frac{(-1)^{k}x^{k}}{k+1}=1-frac{x}{2}+frac{x^2}{3}-...$$
$$frac{d}{dx}Bigg(frac{ln(1+x)}{x}Bigg)=frac{d}{dx}bigg(1-frac{x}{2}+frac{x^2}{3}-...bigg)=-frac{1}{2}+frac{2x}{3}-frac{3x^2}{4}+...$$
So,
$$lim_{xto0}Bigg(frac{d}{dx}Bigg(frac{ln(1+x)}{x}Bigg)Bigg)=lim_{xto0}bigg(-frac{1}{2}+frac{2x}{3}-frac{3x^2}{4}+...bigg)=fbox{$-frac{1}{2}$}$$
$endgroup$
add a comment |
$begingroup$
Your original function $f(x)=frac{ln(1+x)}{x}$ can be written using the taylor series expansion of $ln(1+x)$ as follows:
$$ln(1+x)=sum_{k=1}^{infty}frac{(-1)^{k+1}x^{k}}{k}=x-frac{x^2}{2}+frac{x^3}{3}-...$$
$$thereforefrac{ln(1+x)}{x}=sum_{k=1}^{infty}frac{(-1)^{k+1}x^{k-1}}{k}=sum_{k=0}^{infty}frac{(-1)^{k}x^{k}}{k+1}=1-frac{x}{2}+frac{x^2}{3}-...$$
$$frac{d}{dx}Bigg(frac{ln(1+x)}{x}Bigg)=frac{d}{dx}bigg(1-frac{x}{2}+frac{x^2}{3}-...bigg)=-frac{1}{2}+frac{2x}{3}-frac{3x^2}{4}+...$$
So,
$$lim_{xto0}Bigg(frac{d}{dx}Bigg(frac{ln(1+x)}{x}Bigg)Bigg)=lim_{xto0}bigg(-frac{1}{2}+frac{2x}{3}-frac{3x^2}{4}+...bigg)=fbox{$-frac{1}{2}$}$$
$endgroup$
add a comment |
$begingroup$
Your original function $f(x)=frac{ln(1+x)}{x}$ can be written using the taylor series expansion of $ln(1+x)$ as follows:
$$ln(1+x)=sum_{k=1}^{infty}frac{(-1)^{k+1}x^{k}}{k}=x-frac{x^2}{2}+frac{x^3}{3}-...$$
$$thereforefrac{ln(1+x)}{x}=sum_{k=1}^{infty}frac{(-1)^{k+1}x^{k-1}}{k}=sum_{k=0}^{infty}frac{(-1)^{k}x^{k}}{k+1}=1-frac{x}{2}+frac{x^2}{3}-...$$
$$frac{d}{dx}Bigg(frac{ln(1+x)}{x}Bigg)=frac{d}{dx}bigg(1-frac{x}{2}+frac{x^2}{3}-...bigg)=-frac{1}{2}+frac{2x}{3}-frac{3x^2}{4}+...$$
So,
$$lim_{xto0}Bigg(frac{d}{dx}Bigg(frac{ln(1+x)}{x}Bigg)Bigg)=lim_{xto0}bigg(-frac{1}{2}+frac{2x}{3}-frac{3x^2}{4}+...bigg)=fbox{$-frac{1}{2}$}$$
$endgroup$
Your original function $f(x)=frac{ln(1+x)}{x}$ can be written using the taylor series expansion of $ln(1+x)$ as follows:
$$ln(1+x)=sum_{k=1}^{infty}frac{(-1)^{k+1}x^{k}}{k}=x-frac{x^2}{2}+frac{x^3}{3}-...$$
$$thereforefrac{ln(1+x)}{x}=sum_{k=1}^{infty}frac{(-1)^{k+1}x^{k-1}}{k}=sum_{k=0}^{infty}frac{(-1)^{k}x^{k}}{k+1}=1-frac{x}{2}+frac{x^2}{3}-...$$
$$frac{d}{dx}Bigg(frac{ln(1+x)}{x}Bigg)=frac{d}{dx}bigg(1-frac{x}{2}+frac{x^2}{3}-...bigg)=-frac{1}{2}+frac{2x}{3}-frac{3x^2}{4}+...$$
So,
$$lim_{xto0}Bigg(frac{d}{dx}Bigg(frac{ln(1+x)}{x}Bigg)Bigg)=lim_{xto0}bigg(-frac{1}{2}+frac{2x}{3}-frac{3x^2}{4}+...bigg)=fbox{$-frac{1}{2}$}$$
answered Jan 5 at 0:17
Peter ForemanPeter Foreman
5,8141317
5,8141317
add a comment |
add a comment |
$begingroup$
$frac {1}{x(x+1)} - frac {ln(x+1)}{x^2} = frac {d}{dx} frac {ln(x+1)}{x}$
At which point I am inclined to use a series
$frac {d}{dx} (1 - frac x2 + cdots)$
as $x$ goes to $0$ equals $-frac 12$
Without using a series, I have.
$frac {d}{dx} frac {ln(x+1)}{x}$ evaluated at $x = 0$
is $lim_limits{hto 0} frac{ ln(1+h) - 1}{h^2}$
$endgroup$
$begingroup$
Yes I can't use that too, edited the post, but thanks!
$endgroup$
– Heryon
Jan 5 at 0:33
add a comment |
$begingroup$
$frac {1}{x(x+1)} - frac {ln(x+1)}{x^2} = frac {d}{dx} frac {ln(x+1)}{x}$
At which point I am inclined to use a series
$frac {d}{dx} (1 - frac x2 + cdots)$
as $x$ goes to $0$ equals $-frac 12$
Without using a series, I have.
$frac {d}{dx} frac {ln(x+1)}{x}$ evaluated at $x = 0$
is $lim_limits{hto 0} frac{ ln(1+h) - 1}{h^2}$
$endgroup$
$begingroup$
Yes I can't use that too, edited the post, but thanks!
$endgroup$
– Heryon
Jan 5 at 0:33
add a comment |
$begingroup$
$frac {1}{x(x+1)} - frac {ln(x+1)}{x^2} = frac {d}{dx} frac {ln(x+1)}{x}$
At which point I am inclined to use a series
$frac {d}{dx} (1 - frac x2 + cdots)$
as $x$ goes to $0$ equals $-frac 12$
Without using a series, I have.
$frac {d}{dx} frac {ln(x+1)}{x}$ evaluated at $x = 0$
is $lim_limits{hto 0} frac{ ln(1+h) - 1}{h^2}$
$endgroup$
$frac {1}{x(x+1)} - frac {ln(x+1)}{x^2} = frac {d}{dx} frac {ln(x+1)}{x}$
At which point I am inclined to use a series
$frac {d}{dx} (1 - frac x2 + cdots)$
as $x$ goes to $0$ equals $-frac 12$
Without using a series, I have.
$frac {d}{dx} frac {ln(x+1)}{x}$ evaluated at $x = 0$
is $lim_limits{hto 0} frac{ ln(1+h) - 1}{h^2}$
edited Jan 5 at 1:30
answered Jan 5 at 0:27
Doug MDoug M
45.3k31954
45.3k31954
$begingroup$
Yes I can't use that too, edited the post, but thanks!
$endgroup$
– Heryon
Jan 5 at 0:33
add a comment |
$begingroup$
Yes I can't use that too, edited the post, but thanks!
$endgroup$
– Heryon
Jan 5 at 0:33
$begingroup$
Yes I can't use that too, edited the post, but thanks!
$endgroup$
– Heryon
Jan 5 at 0:33
$begingroup$
Yes I can't use that too, edited the post, but thanks!
$endgroup$
– Heryon
Jan 5 at 0:33
add a comment |
$begingroup$
Because $frac{x}{1+x}<ln(1+x)<x$
So when $ x rightarrow 0+, f(x) rightarrow 1$
When $xrightarrow0-$ is similar
$endgroup$
add a comment |
$begingroup$
Because $frac{x}{1+x}<ln(1+x)<x$
So when $ x rightarrow 0+, f(x) rightarrow 1$
When $xrightarrow0-$ is similar
$endgroup$
add a comment |
$begingroup$
Because $frac{x}{1+x}<ln(1+x)<x$
So when $ x rightarrow 0+, f(x) rightarrow 1$
When $xrightarrow0-$ is similar
$endgroup$
Because $frac{x}{1+x}<ln(1+x)<x$
So when $ x rightarrow 0+, f(x) rightarrow 1$
When $xrightarrow0-$ is similar
answered Jan 5 at 1:46
李子镔李子镔
314
314
add a comment |
add a comment |
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