How can I claim a one-sided limit doesn't exist?












4












$begingroup$


I have to find the limit $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}} $$



Now, I tried using L'Hôpital's rule, but it doesn't lead anywhere.



Manually trying to convert the functions to another form doesn't seem to go anywhere either, so I determined that the limit must be undefined.



However, I cannot prove it. What can I do?










share|cite|improve this question











$endgroup$








  • 2




    $begingroup$
    Using Taylor, the numerator is $2xcdot x+o(x^3)$ and the limit is $0$.
    $endgroup$
    – Yves Daoust
    Jan 30 at 16:37










  • $begingroup$
    We haven't been taught about Taylor yet, I should add this to my question. However, thanks for the answer, i'll research Taylor on my own.
    $endgroup$
    – sdds
    Jan 30 at 16:40












  • $begingroup$
    To answer the question in the title, see for example here
    $endgroup$
    – BlueRaja - Danny Pflughoeft
    Jan 30 at 21:37










  • $begingroup$
    After you ask a question here, if you get an acceptable answer, you should "accept" the answer by clicking the check mark $checkmark$ next to it. This scores points for you and for the person who answered your question. You can find out more about accepting answers here: How do I accept an answer?, Why should we accept answers?, What should I do if someone answers my question?.
    $endgroup$
    – Clement C.
    Feb 3 at 23:26
















4












$begingroup$


I have to find the limit $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}} $$



Now, I tried using L'Hôpital's rule, but it doesn't lead anywhere.



Manually trying to convert the functions to another form doesn't seem to go anywhere either, so I determined that the limit must be undefined.



However, I cannot prove it. What can I do?










share|cite|improve this question











$endgroup$








  • 2




    $begingroup$
    Using Taylor, the numerator is $2xcdot x+o(x^3)$ and the limit is $0$.
    $endgroup$
    – Yves Daoust
    Jan 30 at 16:37










  • $begingroup$
    We haven't been taught about Taylor yet, I should add this to my question. However, thanks for the answer, i'll research Taylor on my own.
    $endgroup$
    – sdds
    Jan 30 at 16:40












  • $begingroup$
    To answer the question in the title, see for example here
    $endgroup$
    – BlueRaja - Danny Pflughoeft
    Jan 30 at 21:37










  • $begingroup$
    After you ask a question here, if you get an acceptable answer, you should "accept" the answer by clicking the check mark $checkmark$ next to it. This scores points for you and for the person who answered your question. You can find out more about accepting answers here: How do I accept an answer?, Why should we accept answers?, What should I do if someone answers my question?.
    $endgroup$
    – Clement C.
    Feb 3 at 23:26














4












4








4





$begingroup$


I have to find the limit $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}} $$



Now, I tried using L'Hôpital's rule, but it doesn't lead anywhere.



Manually trying to convert the functions to another form doesn't seem to go anywhere either, so I determined that the limit must be undefined.



However, I cannot prove it. What can I do?










share|cite|improve this question











$endgroup$




I have to find the limit $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}} $$



Now, I tried using L'Hôpital's rule, but it doesn't lead anywhere.



Manually trying to convert the functions to another form doesn't seem to go anywhere either, so I determined that the limit must be undefined.



However, I cannot prove it. What can I do?







real-analysis limits logarithms proof-explanation limits-without-lhopital






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Feb 28 at 7:46









Michael Rozenberg

109k1896201




109k1896201










asked Jan 30 at 16:21









sddssdds

296




296








  • 2




    $begingroup$
    Using Taylor, the numerator is $2xcdot x+o(x^3)$ and the limit is $0$.
    $endgroup$
    – Yves Daoust
    Jan 30 at 16:37










  • $begingroup$
    We haven't been taught about Taylor yet, I should add this to my question. However, thanks for the answer, i'll research Taylor on my own.
    $endgroup$
    – sdds
    Jan 30 at 16:40












  • $begingroup$
    To answer the question in the title, see for example here
    $endgroup$
    – BlueRaja - Danny Pflughoeft
    Jan 30 at 21:37










  • $begingroup$
    After you ask a question here, if you get an acceptable answer, you should "accept" the answer by clicking the check mark $checkmark$ next to it. This scores points for you and for the person who answered your question. You can find out more about accepting answers here: How do I accept an answer?, Why should we accept answers?, What should I do if someone answers my question?.
    $endgroup$
    – Clement C.
    Feb 3 at 23:26














  • 2




    $begingroup$
    Using Taylor, the numerator is $2xcdot x+o(x^3)$ and the limit is $0$.
    $endgroup$
    – Yves Daoust
    Jan 30 at 16:37










  • $begingroup$
    We haven't been taught about Taylor yet, I should add this to my question. However, thanks for the answer, i'll research Taylor on my own.
    $endgroup$
    – sdds
    Jan 30 at 16:40












  • $begingroup$
    To answer the question in the title, see for example here
    $endgroup$
    – BlueRaja - Danny Pflughoeft
    Jan 30 at 21:37










  • $begingroup$
    After you ask a question here, if you get an acceptable answer, you should "accept" the answer by clicking the check mark $checkmark$ next to it. This scores points for you and for the person who answered your question. You can find out more about accepting answers here: How do I accept an answer?, Why should we accept answers?, What should I do if someone answers my question?.
    $endgroup$
    – Clement C.
    Feb 3 at 23:26








2




2




$begingroup$
Using Taylor, the numerator is $2xcdot x+o(x^3)$ and the limit is $0$.
$endgroup$
– Yves Daoust
Jan 30 at 16:37




$begingroup$
Using Taylor, the numerator is $2xcdot x+o(x^3)$ and the limit is $0$.
$endgroup$
– Yves Daoust
Jan 30 at 16:37












$begingroup$
We haven't been taught about Taylor yet, I should add this to my question. However, thanks for the answer, i'll research Taylor on my own.
$endgroup$
– sdds
Jan 30 at 16:40






$begingroup$
We haven't been taught about Taylor yet, I should add this to my question. However, thanks for the answer, i'll research Taylor on my own.
$endgroup$
– sdds
Jan 30 at 16:40














$begingroup$
To answer the question in the title, see for example here
$endgroup$
– BlueRaja - Danny Pflughoeft
Jan 30 at 21:37




$begingroup$
To answer the question in the title, see for example here
$endgroup$
– BlueRaja - Danny Pflughoeft
Jan 30 at 21:37












$begingroup$
After you ask a question here, if you get an acceptable answer, you should "accept" the answer by clicking the check mark $checkmark$ next to it. This scores points for you and for the person who answered your question. You can find out more about accepting answers here: How do I accept an answer?, Why should we accept answers?, What should I do if someone answers my question?.
$endgroup$
– Clement C.
Feb 3 at 23:26




$begingroup$
After you ask a question here, if you get an acceptable answer, you should "accept" the answer by clicking the check mark $checkmark$ next to it. This scores points for you and for the person who answered your question. You can find out more about accepting answers here: How do I accept an answer?, Why should we accept answers?, What should I do if someone answers my question?.
$endgroup$
– Clement C.
Feb 3 at 23:26










4 Answers
4






active

oldest

votes


















13












$begingroup$

Hint:



Use the facts that (1) $lim_{xto 0} frac{ln(1+2x)}{2x} = 1$, (2) $lim_{xto 0} frac{sin x}{x} = 1$, and (3) $lim_{xto 0^+} frac{x^2}{sqrt{x^3}} = 0$ to show the limit is $2cdot 1cdot 0 = 0$.



To do so, rewrite, for $x>0$,
$$
frac{ln(1+2x)sin x}{sqrt{x^3}}=2cdot frac{ln(1+2x)}{2x}cdotfrac{sin x}{x}cdotfrac{x^2}{sqrt{x^3}}
$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Thank you, this conversion never occurred to me
    $endgroup$
    – sdds
    Jan 30 at 16:37










  • $begingroup$
    @sdds You're welcome!
    $endgroup$
    – Clement C.
    Jan 30 at 16:40



















11












$begingroup$

$$frac{ln(1+2x)sin{x}}{sqrt{x^3}}=frac{ln(1+2x)}{2x}cdotfrac{sin{x}}{x}cdot2sqrt{x}rightarrow0.$$






share|cite|improve this answer









$endgroup$





















    3












    $begingroup$

    It is known that



    $$lim_{xto 0^+}frac{sin x}x=1$$ and $$lim_{xto 0^+}frac{log(1+x)}x=1=lim_{xto 0^+}frac{log(1+2x)}{2x}.$$



    Then



    $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}}=1cdot2cdotlim_{xto 0^+} sqrt x.$$






    share|cite|improve this answer









    $endgroup$









    • 2




      $begingroup$
      It's true, but how is that different from the current two answers?
      $endgroup$
      – Clement C.
      Jan 30 at 16:45



















    0












    $begingroup$

    If you want to, you should apply L'Hospital's rule twice:
    $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}}=\
    lim_{xto 0^+} frac{frac{2}{1+2x}sin x+ln(1+2x)cos x}{1.5sqrt {x}}=\
    lim_{xto 0^+} frac{frac{-4}{(1+2x)^2}sin x+frac{2}{1+2x}cos x+frac{2}{1+2x}cos x-ln(1+2x)sin x}{frac{0.75}{sqrt {x}}}=0.
    $$






    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









      13












      $begingroup$

      Hint:



      Use the facts that (1) $lim_{xto 0} frac{ln(1+2x)}{2x} = 1$, (2) $lim_{xto 0} frac{sin x}{x} = 1$, and (3) $lim_{xto 0^+} frac{x^2}{sqrt{x^3}} = 0$ to show the limit is $2cdot 1cdot 0 = 0$.



      To do so, rewrite, for $x>0$,
      $$
      frac{ln(1+2x)sin x}{sqrt{x^3}}=2cdot frac{ln(1+2x)}{2x}cdotfrac{sin x}{x}cdotfrac{x^2}{sqrt{x^3}}
      $$






      share|cite|improve this answer









      $endgroup$













      • $begingroup$
        Thank you, this conversion never occurred to me
        $endgroup$
        – sdds
        Jan 30 at 16:37










      • $begingroup$
        @sdds You're welcome!
        $endgroup$
        – Clement C.
        Jan 30 at 16:40
















      13












      $begingroup$

      Hint:



      Use the facts that (1) $lim_{xto 0} frac{ln(1+2x)}{2x} = 1$, (2) $lim_{xto 0} frac{sin x}{x} = 1$, and (3) $lim_{xto 0^+} frac{x^2}{sqrt{x^3}} = 0$ to show the limit is $2cdot 1cdot 0 = 0$.



      To do so, rewrite, for $x>0$,
      $$
      frac{ln(1+2x)sin x}{sqrt{x^3}}=2cdot frac{ln(1+2x)}{2x}cdotfrac{sin x}{x}cdotfrac{x^2}{sqrt{x^3}}
      $$






      share|cite|improve this answer









      $endgroup$













      • $begingroup$
        Thank you, this conversion never occurred to me
        $endgroup$
        – sdds
        Jan 30 at 16:37










      • $begingroup$
        @sdds You're welcome!
        $endgroup$
        – Clement C.
        Jan 30 at 16:40














      13












      13








      13





      $begingroup$

      Hint:



      Use the facts that (1) $lim_{xto 0} frac{ln(1+2x)}{2x} = 1$, (2) $lim_{xto 0} frac{sin x}{x} = 1$, and (3) $lim_{xto 0^+} frac{x^2}{sqrt{x^3}} = 0$ to show the limit is $2cdot 1cdot 0 = 0$.



      To do so, rewrite, for $x>0$,
      $$
      frac{ln(1+2x)sin x}{sqrt{x^3}}=2cdot frac{ln(1+2x)}{2x}cdotfrac{sin x}{x}cdotfrac{x^2}{sqrt{x^3}}
      $$






      share|cite|improve this answer









      $endgroup$



      Hint:



      Use the facts that (1) $lim_{xto 0} frac{ln(1+2x)}{2x} = 1$, (2) $lim_{xto 0} frac{sin x}{x} = 1$, and (3) $lim_{xto 0^+} frac{x^2}{sqrt{x^3}} = 0$ to show the limit is $2cdot 1cdot 0 = 0$.



      To do so, rewrite, for $x>0$,
      $$
      frac{ln(1+2x)sin x}{sqrt{x^3}}=2cdot frac{ln(1+2x)}{2x}cdotfrac{sin x}{x}cdotfrac{x^2}{sqrt{x^3}}
      $$







      share|cite|improve this answer












      share|cite|improve this answer



      share|cite|improve this answer










      answered Jan 30 at 16:24









      Clement C.Clement C.

      51k34093




      51k34093












      • $begingroup$
        Thank you, this conversion never occurred to me
        $endgroup$
        – sdds
        Jan 30 at 16:37










      • $begingroup$
        @sdds You're welcome!
        $endgroup$
        – Clement C.
        Jan 30 at 16:40


















      • $begingroup$
        Thank you, this conversion never occurred to me
        $endgroup$
        – sdds
        Jan 30 at 16:37










      • $begingroup$
        @sdds You're welcome!
        $endgroup$
        – Clement C.
        Jan 30 at 16:40
















      $begingroup$
      Thank you, this conversion never occurred to me
      $endgroup$
      – sdds
      Jan 30 at 16:37




      $begingroup$
      Thank you, this conversion never occurred to me
      $endgroup$
      – sdds
      Jan 30 at 16:37












      $begingroup$
      @sdds You're welcome!
      $endgroup$
      – Clement C.
      Jan 30 at 16:40




      $begingroup$
      @sdds You're welcome!
      $endgroup$
      – Clement C.
      Jan 30 at 16:40











      11












      $begingroup$

      $$frac{ln(1+2x)sin{x}}{sqrt{x^3}}=frac{ln(1+2x)}{2x}cdotfrac{sin{x}}{x}cdot2sqrt{x}rightarrow0.$$






      share|cite|improve this answer









      $endgroup$


















        11












        $begingroup$

        $$frac{ln(1+2x)sin{x}}{sqrt{x^3}}=frac{ln(1+2x)}{2x}cdotfrac{sin{x}}{x}cdot2sqrt{x}rightarrow0.$$






        share|cite|improve this answer









        $endgroup$
















          11












          11








          11





          $begingroup$

          $$frac{ln(1+2x)sin{x}}{sqrt{x^3}}=frac{ln(1+2x)}{2x}cdotfrac{sin{x}}{x}cdot2sqrt{x}rightarrow0.$$






          share|cite|improve this answer









          $endgroup$



          $$frac{ln(1+2x)sin{x}}{sqrt{x^3}}=frac{ln(1+2x)}{2x}cdotfrac{sin{x}}{x}cdot2sqrt{x}rightarrow0.$$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Jan 30 at 16:24









          Michael RozenbergMichael Rozenberg

          109k1896201




          109k1896201























              3












              $begingroup$

              It is known that



              $$lim_{xto 0^+}frac{sin x}x=1$$ and $$lim_{xto 0^+}frac{log(1+x)}x=1=lim_{xto 0^+}frac{log(1+2x)}{2x}.$$



              Then



              $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}}=1cdot2cdotlim_{xto 0^+} sqrt x.$$






              share|cite|improve this answer









              $endgroup$









              • 2




                $begingroup$
                It's true, but how is that different from the current two answers?
                $endgroup$
                – Clement C.
                Jan 30 at 16:45
















              3












              $begingroup$

              It is known that



              $$lim_{xto 0^+}frac{sin x}x=1$$ and $$lim_{xto 0^+}frac{log(1+x)}x=1=lim_{xto 0^+}frac{log(1+2x)}{2x}.$$



              Then



              $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}}=1cdot2cdotlim_{xto 0^+} sqrt x.$$






              share|cite|improve this answer









              $endgroup$









              • 2




                $begingroup$
                It's true, but how is that different from the current two answers?
                $endgroup$
                – Clement C.
                Jan 30 at 16:45














              3












              3








              3





              $begingroup$

              It is known that



              $$lim_{xto 0^+}frac{sin x}x=1$$ and $$lim_{xto 0^+}frac{log(1+x)}x=1=lim_{xto 0^+}frac{log(1+2x)}{2x}.$$



              Then



              $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}}=1cdot2cdotlim_{xto 0^+} sqrt x.$$






              share|cite|improve this answer









              $endgroup$



              It is known that



              $$lim_{xto 0^+}frac{sin x}x=1$$ and $$lim_{xto 0^+}frac{log(1+x)}x=1=lim_{xto 0^+}frac{log(1+2x)}{2x}.$$



              Then



              $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}}=1cdot2cdotlim_{xto 0^+} sqrt x.$$







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered Jan 30 at 16:42









              Yves DaoustYves Daoust

              132k676229




              132k676229








              • 2




                $begingroup$
                It's true, but how is that different from the current two answers?
                $endgroup$
                – Clement C.
                Jan 30 at 16:45














              • 2




                $begingroup$
                It's true, but how is that different from the current two answers?
                $endgroup$
                – Clement C.
                Jan 30 at 16:45








              2




              2




              $begingroup$
              It's true, but how is that different from the current two answers?
              $endgroup$
              – Clement C.
              Jan 30 at 16:45




              $begingroup$
              It's true, but how is that different from the current two answers?
              $endgroup$
              – Clement C.
              Jan 30 at 16:45











              0












              $begingroup$

              If you want to, you should apply L'Hospital's rule twice:
              $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}}=\
              lim_{xto 0^+} frac{frac{2}{1+2x}sin x+ln(1+2x)cos x}{1.5sqrt {x}}=\
              lim_{xto 0^+} frac{frac{-4}{(1+2x)^2}sin x+frac{2}{1+2x}cos x+frac{2}{1+2x}cos x-ln(1+2x)sin x}{frac{0.75}{sqrt {x}}}=0.
              $$






              share|cite|improve this answer









              $endgroup$


















                0












                $begingroup$

                If you want to, you should apply L'Hospital's rule twice:
                $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}}=\
                lim_{xto 0^+} frac{frac{2}{1+2x}sin x+ln(1+2x)cos x}{1.5sqrt {x}}=\
                lim_{xto 0^+} frac{frac{-4}{(1+2x)^2}sin x+frac{2}{1+2x}cos x+frac{2}{1+2x}cos x-ln(1+2x)sin x}{frac{0.75}{sqrt {x}}}=0.
                $$






                share|cite|improve this answer









                $endgroup$
















                  0












                  0








                  0





                  $begingroup$

                  If you want to, you should apply L'Hospital's rule twice:
                  $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}}=\
                  lim_{xto 0^+} frac{frac{2}{1+2x}sin x+ln(1+2x)cos x}{1.5sqrt {x}}=\
                  lim_{xto 0^+} frac{frac{-4}{(1+2x)^2}sin x+frac{2}{1+2x}cos x+frac{2}{1+2x}cos x-ln(1+2x)sin x}{frac{0.75}{sqrt {x}}}=0.
                  $$






                  share|cite|improve this answer









                  $endgroup$



                  If you want to, you should apply L'Hospital's rule twice:
                  $$lim_{xto 0^+} frac{ln(1+2x)sin x}{sqrt {x^3}}=\
                  lim_{xto 0^+} frac{frac{2}{1+2x}sin x+ln(1+2x)cos x}{1.5sqrt {x}}=\
                  lim_{xto 0^+} frac{frac{-4}{(1+2x)^2}sin x+frac{2}{1+2x}cos x+frac{2}{1+2x}cos x-ln(1+2x)sin x}{frac{0.75}{sqrt {x}}}=0.
                  $$







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Feb 28 at 10:15









                  farruhotafarruhota

                  21.8k2842




                  21.8k2842






























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