Bounds of $frac{ln(x+1)}{x} forall x>0$
$begingroup$
$f:(0,infty)$,
$f(x)=frac{ln(x+1)}{x}$
Prove that for $forall x>0$ that $f(x) in(0,1)$. I calculated the derivative of $f(x)$: $f'(x)=frac{frac{x}{x+1}-ln(1+x)}{x^2}$ which I think simplifies to $frac{x^3}{x+1}-x^2(ln(1+x))$. I have no idea what to do next, I can't find the roots of this equation and I don't see any connection as of why it should be bounded by 0 and 1.
I hope I formatted this well, I don't usually post here but I am really curious how could I solve this kind of exercise.
calculus
$endgroup$
add a comment |
$begingroup$
$f:(0,infty)$,
$f(x)=frac{ln(x+1)}{x}$
Prove that for $forall x>0$ that $f(x) in(0,1)$. I calculated the derivative of $f(x)$: $f'(x)=frac{frac{x}{x+1}-ln(1+x)}{x^2}$ which I think simplifies to $frac{x^3}{x+1}-x^2(ln(1+x))$. I have no idea what to do next, I can't find the roots of this equation and I don't see any connection as of why it should be bounded by 0 and 1.
I hope I formatted this well, I don't usually post here but I am really curious how could I solve this kind of exercise.
calculus
$endgroup$
2
$begingroup$
$0<frac{ln(x+1)}{x}<1$ $iff$ $0<ln(x+1)<x$.
$endgroup$
– A.Γ.
Dec 29 '18 at 20:24
$begingroup$
Note that if $x>0$ then $e^x>x+1$ by a simple Taylor expansion. This proves what you want
$endgroup$
– Shalop
Dec 29 '18 at 20:43
$begingroup$
@A.Γ. Wow I never thought to work from the statement that I have to prove. It almost seems too easy to be true. I can't thank you enough for this answer.
$endgroup$
– Radu Gabriel
Dec 29 '18 at 21:13
add a comment |
$begingroup$
$f:(0,infty)$,
$f(x)=frac{ln(x+1)}{x}$
Prove that for $forall x>0$ that $f(x) in(0,1)$. I calculated the derivative of $f(x)$: $f'(x)=frac{frac{x}{x+1}-ln(1+x)}{x^2}$ which I think simplifies to $frac{x^3}{x+1}-x^2(ln(1+x))$. I have no idea what to do next, I can't find the roots of this equation and I don't see any connection as of why it should be bounded by 0 and 1.
I hope I formatted this well, I don't usually post here but I am really curious how could I solve this kind of exercise.
calculus
$endgroup$
$f:(0,infty)$,
$f(x)=frac{ln(x+1)}{x}$
Prove that for $forall x>0$ that $f(x) in(0,1)$. I calculated the derivative of $f(x)$: $f'(x)=frac{frac{x}{x+1}-ln(1+x)}{x^2}$ which I think simplifies to $frac{x^3}{x+1}-x^2(ln(1+x))$. I have no idea what to do next, I can't find the roots of this equation and I don't see any connection as of why it should be bounded by 0 and 1.
I hope I formatted this well, I don't usually post here but I am really curious how could I solve this kind of exercise.
calculus
calculus
edited Dec 29 '18 at 20:28
A.Γ.
22.9k32656
22.9k32656
asked Dec 29 '18 at 20:21
Radu GabrielRadu Gabriel
1115
1115
2
$begingroup$
$0<frac{ln(x+1)}{x}<1$ $iff$ $0<ln(x+1)<x$.
$endgroup$
– A.Γ.
Dec 29 '18 at 20:24
$begingroup$
Note that if $x>0$ then $e^x>x+1$ by a simple Taylor expansion. This proves what you want
$endgroup$
– Shalop
Dec 29 '18 at 20:43
$begingroup$
@A.Γ. Wow I never thought to work from the statement that I have to prove. It almost seems too easy to be true. I can't thank you enough for this answer.
$endgroup$
– Radu Gabriel
Dec 29 '18 at 21:13
add a comment |
2
$begingroup$
$0<frac{ln(x+1)}{x}<1$ $iff$ $0<ln(x+1)<x$.
$endgroup$
– A.Γ.
Dec 29 '18 at 20:24
$begingroup$
Note that if $x>0$ then $e^x>x+1$ by a simple Taylor expansion. This proves what you want
$endgroup$
– Shalop
Dec 29 '18 at 20:43
$begingroup$
@A.Γ. Wow I never thought to work from the statement that I have to prove. It almost seems too easy to be true. I can't thank you enough for this answer.
$endgroup$
– Radu Gabriel
Dec 29 '18 at 21:13
2
2
$begingroup$
$0<frac{ln(x+1)}{x}<1$ $iff$ $0<ln(x+1)<x$.
$endgroup$
– A.Γ.
Dec 29 '18 at 20:24
$begingroup$
$0<frac{ln(x+1)}{x}<1$ $iff$ $0<ln(x+1)<x$.
$endgroup$
– A.Γ.
Dec 29 '18 at 20:24
$begingroup$
Note that if $x>0$ then $e^x>x+1$ by a simple Taylor expansion. This proves what you want
$endgroup$
– Shalop
Dec 29 '18 at 20:43
$begingroup$
Note that if $x>0$ then $e^x>x+1$ by a simple Taylor expansion. This proves what you want
$endgroup$
– Shalop
Dec 29 '18 at 20:43
$begingroup$
@A.Γ. Wow I never thought to work from the statement that I have to prove. It almost seems too easy to be true. I can't thank you enough for this answer.
$endgroup$
– Radu Gabriel
Dec 29 '18 at 21:13
$begingroup$
@A.Γ. Wow I never thought to work from the statement that I have to prove. It almost seems too easy to be true. I can't thank you enough for this answer.
$endgroup$
– Radu Gabriel
Dec 29 '18 at 21:13
add a comment |
4 Answers
4
active
oldest
votes
$begingroup$
First of all if is easy to see that $dfrac{ln{(x+1)}}{x}>0$.
It's well known that $ln{x}leq x-1$ where the equality is valid only for $x=1$. (You can prove it if you define the function $f(x)=ln{x}-x+1$ and find the global maximum using simple calculus). Then by this we get that $ln{(x+1)}leq x$ so for $x>0$ we get $dfrac{ln{(x+1)}}{x}<1$.
Alexandros
$endgroup$
add a comment |
$begingroup$
For $x in (0,infty)$, you have
$$ln (x+1) = int_0^x frac{dt}{1+t}$$
Hence
$$0 le f(x) = frac{1}{x}int_0^x frac{dt}{1+t} < frac{1}{x}int_0^x dt =1$$
As all considered maps are continuous.
$endgroup$
$begingroup$
Why do you argue for $xin (0,1)$? The domain of the function is $(0, infty)$. Also, what you show seems to be insufficient to prove the desired inequalities.
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:29
$begingroup$
No, it is to be proven on the set of all positive reals (and it is true there). Also, as I said, your argument is not sufficient to show the upper estimation (even on the unit interval).
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:34
$begingroup$
@A.Pongrácz You’re right. Modified the answer.
$endgroup$
– mathcounterexamples.net
Dec 29 '18 at 20:34
add a comment |
$begingroup$
$ln(x+1)$ is a concave function, so the curve is under each of it tangents. Now it happens that the line with equation $y=x$ is its tangent at origin. So for any $x 0$ of its domain, $ln(x+1)le x$. Furthermore, considering the function $x-ln(x+1)$, it is easy to see this function is increasing on [0,+infty), so
$$ln(x+1)<xenspaceforall x>0iff frac{ln(x+1)}x<1enspaceforall x>0.$$
The inequality $dfrac{ln(x+1)}x>0$ is obvious since $1+x>1$.
$endgroup$
add a comment |
$begingroup$
It is much simpler to show that the derivative of $ln(x+1)$ is always in $(0,1)$ (and it is continuous).
Then $ln(x+1)$ is strictly between $0$ and $x$, think about it.
$endgroup$
add a comment |
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
First of all if is easy to see that $dfrac{ln{(x+1)}}{x}>0$.
It's well known that $ln{x}leq x-1$ where the equality is valid only for $x=1$. (You can prove it if you define the function $f(x)=ln{x}-x+1$ and find the global maximum using simple calculus). Then by this we get that $ln{(x+1)}leq x$ so for $x>0$ we get $dfrac{ln{(x+1)}}{x}<1$.
Alexandros
$endgroup$
add a comment |
$begingroup$
First of all if is easy to see that $dfrac{ln{(x+1)}}{x}>0$.
It's well known that $ln{x}leq x-1$ where the equality is valid only for $x=1$. (You can prove it if you define the function $f(x)=ln{x}-x+1$ and find the global maximum using simple calculus). Then by this we get that $ln{(x+1)}leq x$ so for $x>0$ we get $dfrac{ln{(x+1)}}{x}<1$.
Alexandros
$endgroup$
add a comment |
$begingroup$
First of all if is easy to see that $dfrac{ln{(x+1)}}{x}>0$.
It's well known that $ln{x}leq x-1$ where the equality is valid only for $x=1$. (You can prove it if you define the function $f(x)=ln{x}-x+1$ and find the global maximum using simple calculus). Then by this we get that $ln{(x+1)}leq x$ so for $x>0$ we get $dfrac{ln{(x+1)}}{x}<1$.
Alexandros
$endgroup$
First of all if is easy to see that $dfrac{ln{(x+1)}}{x}>0$.
It's well known that $ln{x}leq x-1$ where the equality is valid only for $x=1$. (You can prove it if you define the function $f(x)=ln{x}-x+1$ and find the global maximum using simple calculus). Then by this we get that $ln{(x+1)}leq x$ so for $x>0$ we get $dfrac{ln{(x+1)}}{x}<1$.
Alexandros
answered Dec 29 '18 at 20:34
AlexSygelakisAlexSygelakis
11
11
add a comment |
add a comment |
$begingroup$
For $x in (0,infty)$, you have
$$ln (x+1) = int_0^x frac{dt}{1+t}$$
Hence
$$0 le f(x) = frac{1}{x}int_0^x frac{dt}{1+t} < frac{1}{x}int_0^x dt =1$$
As all considered maps are continuous.
$endgroup$
$begingroup$
Why do you argue for $xin (0,1)$? The domain of the function is $(0, infty)$. Also, what you show seems to be insufficient to prove the desired inequalities.
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:29
$begingroup$
No, it is to be proven on the set of all positive reals (and it is true there). Also, as I said, your argument is not sufficient to show the upper estimation (even on the unit interval).
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:34
$begingroup$
@A.Pongrácz You’re right. Modified the answer.
$endgroup$
– mathcounterexamples.net
Dec 29 '18 at 20:34
add a comment |
$begingroup$
For $x in (0,infty)$, you have
$$ln (x+1) = int_0^x frac{dt}{1+t}$$
Hence
$$0 le f(x) = frac{1}{x}int_0^x frac{dt}{1+t} < frac{1}{x}int_0^x dt =1$$
As all considered maps are continuous.
$endgroup$
$begingroup$
Why do you argue for $xin (0,1)$? The domain of the function is $(0, infty)$. Also, what you show seems to be insufficient to prove the desired inequalities.
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:29
$begingroup$
No, it is to be proven on the set of all positive reals (and it is true there). Also, as I said, your argument is not sufficient to show the upper estimation (even on the unit interval).
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:34
$begingroup$
@A.Pongrácz You’re right. Modified the answer.
$endgroup$
– mathcounterexamples.net
Dec 29 '18 at 20:34
add a comment |
$begingroup$
For $x in (0,infty)$, you have
$$ln (x+1) = int_0^x frac{dt}{1+t}$$
Hence
$$0 le f(x) = frac{1}{x}int_0^x frac{dt}{1+t} < frac{1}{x}int_0^x dt =1$$
As all considered maps are continuous.
$endgroup$
For $x in (0,infty)$, you have
$$ln (x+1) = int_0^x frac{dt}{1+t}$$
Hence
$$0 le f(x) = frac{1}{x}int_0^x frac{dt}{1+t} < frac{1}{x}int_0^x dt =1$$
As all considered maps are continuous.
edited Dec 29 '18 at 20:40
answered Dec 29 '18 at 20:27
mathcounterexamples.netmathcounterexamples.net
27k22158
27k22158
$begingroup$
Why do you argue for $xin (0,1)$? The domain of the function is $(0, infty)$. Also, what you show seems to be insufficient to prove the desired inequalities.
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:29
$begingroup$
No, it is to be proven on the set of all positive reals (and it is true there). Also, as I said, your argument is not sufficient to show the upper estimation (even on the unit interval).
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:34
$begingroup$
@A.Pongrácz You’re right. Modified the answer.
$endgroup$
– mathcounterexamples.net
Dec 29 '18 at 20:34
add a comment |
$begingroup$
Why do you argue for $xin (0,1)$? The domain of the function is $(0, infty)$. Also, what you show seems to be insufficient to prove the desired inequalities.
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:29
$begingroup$
No, it is to be proven on the set of all positive reals (and it is true there). Also, as I said, your argument is not sufficient to show the upper estimation (even on the unit interval).
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:34
$begingroup$
@A.Pongrácz You’re right. Modified the answer.
$endgroup$
– mathcounterexamples.net
Dec 29 '18 at 20:34
$begingroup$
Why do you argue for $xin (0,1)$? The domain of the function is $(0, infty)$. Also, what you show seems to be insufficient to prove the desired inequalities.
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:29
$begingroup$
Why do you argue for $xin (0,1)$? The domain of the function is $(0, infty)$. Also, what you show seems to be insufficient to prove the desired inequalities.
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:29
$begingroup$
No, it is to be proven on the set of all positive reals (and it is true there). Also, as I said, your argument is not sufficient to show the upper estimation (even on the unit interval).
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:34
$begingroup$
No, it is to be proven on the set of all positive reals (and it is true there). Also, as I said, your argument is not sufficient to show the upper estimation (even on the unit interval).
$endgroup$
– A. Pongrácz
Dec 29 '18 at 20:34
$begingroup$
@A.Pongrácz You’re right. Modified the answer.
$endgroup$
– mathcounterexamples.net
Dec 29 '18 at 20:34
$begingroup$
@A.Pongrácz You’re right. Modified the answer.
$endgroup$
– mathcounterexamples.net
Dec 29 '18 at 20:34
add a comment |
$begingroup$
$ln(x+1)$ is a concave function, so the curve is under each of it tangents. Now it happens that the line with equation $y=x$ is its tangent at origin. So for any $x 0$ of its domain, $ln(x+1)le x$. Furthermore, considering the function $x-ln(x+1)$, it is easy to see this function is increasing on [0,+infty), so
$$ln(x+1)<xenspaceforall x>0iff frac{ln(x+1)}x<1enspaceforall x>0.$$
The inequality $dfrac{ln(x+1)}x>0$ is obvious since $1+x>1$.
$endgroup$
add a comment |
$begingroup$
$ln(x+1)$ is a concave function, so the curve is under each of it tangents. Now it happens that the line with equation $y=x$ is its tangent at origin. So for any $x 0$ of its domain, $ln(x+1)le x$. Furthermore, considering the function $x-ln(x+1)$, it is easy to see this function is increasing on [0,+infty), so
$$ln(x+1)<xenspaceforall x>0iff frac{ln(x+1)}x<1enspaceforall x>0.$$
The inequality $dfrac{ln(x+1)}x>0$ is obvious since $1+x>1$.
$endgroup$
add a comment |
$begingroup$
$ln(x+1)$ is a concave function, so the curve is under each of it tangents. Now it happens that the line with equation $y=x$ is its tangent at origin. So for any $x 0$ of its domain, $ln(x+1)le x$. Furthermore, considering the function $x-ln(x+1)$, it is easy to see this function is increasing on [0,+infty), so
$$ln(x+1)<xenspaceforall x>0iff frac{ln(x+1)}x<1enspaceforall x>0.$$
The inequality $dfrac{ln(x+1)}x>0$ is obvious since $1+x>1$.
$endgroup$
$ln(x+1)$ is a concave function, so the curve is under each of it tangents. Now it happens that the line with equation $y=x$ is its tangent at origin. So for any $x 0$ of its domain, $ln(x+1)le x$. Furthermore, considering the function $x-ln(x+1)$, it is easy to see this function is increasing on [0,+infty), so
$$ln(x+1)<xenspaceforall x>0iff frac{ln(x+1)}x<1enspaceforall x>0.$$
The inequality $dfrac{ln(x+1)}x>0$ is obvious since $1+x>1$.
answered Dec 29 '18 at 20:43
BernardBernard
123k741117
123k741117
add a comment |
add a comment |
$begingroup$
It is much simpler to show that the derivative of $ln(x+1)$ is always in $(0,1)$ (and it is continuous).
Then $ln(x+1)$ is strictly between $0$ and $x$, think about it.
$endgroup$
add a comment |
$begingroup$
It is much simpler to show that the derivative of $ln(x+1)$ is always in $(0,1)$ (and it is continuous).
Then $ln(x+1)$ is strictly between $0$ and $x$, think about it.
$endgroup$
add a comment |
$begingroup$
It is much simpler to show that the derivative of $ln(x+1)$ is always in $(0,1)$ (and it is continuous).
Then $ln(x+1)$ is strictly between $0$ and $x$, think about it.
$endgroup$
It is much simpler to show that the derivative of $ln(x+1)$ is always in $(0,1)$ (and it is continuous).
Then $ln(x+1)$ is strictly between $0$ and $x$, think about it.
answered Dec 29 '18 at 20:27
A. PongráczA. Pongrácz
5,9731929
5,9731929
add a comment |
add a comment |
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2
$begingroup$
$0<frac{ln(x+1)}{x}<1$ $iff$ $0<ln(x+1)<x$.
$endgroup$
– A.Γ.
Dec 29 '18 at 20:24
$begingroup$
Note that if $x>0$ then $e^x>x+1$ by a simple Taylor expansion. This proves what you want
$endgroup$
– Shalop
Dec 29 '18 at 20:43
$begingroup$
@A.Γ. Wow I never thought to work from the statement that I have to prove. It almost seems too easy to be true. I can't thank you enough for this answer.
$endgroup$
– Radu Gabriel
Dec 29 '18 at 21:13