Bounds of $frac{ln(x+1)}{x} forall x>0$












2












$begingroup$


$f:(0,infty)$,
$f(x)=frac{ln(x+1)}{x}$

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.










share|cite|improve this question











$endgroup$








  • 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
















2












$begingroup$


$f:(0,infty)$,
$f(x)=frac{ln(x+1)}{x}$

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.










share|cite|improve this question











$endgroup$








  • 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














2












2








2





$begingroup$


$f:(0,infty)$,
$f(x)=frac{ln(x+1)}{x}$

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.










share|cite|improve this question











$endgroup$




$f:(0,infty)$,
$f(x)=frac{ln(x+1)}{x}$

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






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








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














  • 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










4 Answers
4






active

oldest

votes


















0












$begingroup$

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






share|cite|improve this answer









$endgroup$





















    0












    $begingroup$

    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.






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      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



















    0












    $begingroup$

    $ln(x+1)$ 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 $1+x>1$.






    share|cite|improve this answer









    $endgroup$





















      -1












      $begingroup$

      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.






      share|cite|improve this answer









      $endgroup$













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        4 Answers
        4






        active

        oldest

        votes








        4 Answers
        4






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        0












        $begingroup$

        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






        share|cite|improve this answer









        $endgroup$


















          0












          $begingroup$

          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






          share|cite|improve this answer









          $endgroup$
















            0












            0








            0





            $begingroup$

            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






            share|cite|improve this answer









            $endgroup$



            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







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Dec 29 '18 at 20:34









            AlexSygelakisAlexSygelakis

            11




            11























                0












                $begingroup$

                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.






                share|cite|improve this answer











                $endgroup$













                • $begingroup$
                  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
















                0












                $begingroup$

                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.






                share|cite|improve this answer











                $endgroup$













                • $begingroup$
                  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














                0












                0








                0





                $begingroup$

                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.






                share|cite|improve this answer











                $endgroup$



                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.







                share|cite|improve this answer














                share|cite|improve this answer



                share|cite|improve this answer








                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


















                • $begingroup$
                  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











                0












                $begingroup$

                $ln(x+1)$ 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 $1+x>1$.






                share|cite|improve this answer









                $endgroup$


















                  0












                  $begingroup$

                  $ln(x+1)$ 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 $1+x>1$.






                  share|cite|improve this answer









                  $endgroup$
















                    0












                    0








                    0





                    $begingroup$

                    $ln(x+1)$ 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 $1+x>1$.






                    share|cite|improve this answer









                    $endgroup$



                    $ln(x+1)$ 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 $1+x>1$.







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Dec 29 '18 at 20:43









                    BernardBernard

                    123k741117




                    123k741117























                        -1












                        $begingroup$

                        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.






                        share|cite|improve this answer









                        $endgroup$


















                          -1












                          $begingroup$

                          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.






                          share|cite|improve this answer









                          $endgroup$
















                            -1












                            -1








                            -1





                            $begingroup$

                            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.






                            share|cite|improve this answer









                            $endgroup$



                            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.







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Dec 29 '18 at 20:27









                            A. PongráczA. Pongrácz

                            5,9731929




                            5,9731929






























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