$lim_x to 2 fraccosleft(fracpixright)x-2$ without using De L'Hospital [duplicate]

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  • Limit as $xto 2$ of $fraccos(frac pi x)x-2 $

    5 answers



$$lim_x to 2 fraccosleft(fracpixright)x-2$$



This limit is supposed be found without L'Hospital's Rule, but I have not been able to get close to the answer using conjugates, squares, Pythagorean Identity, half angle formulas or Squeeze Theorem. For each attempt, I expected at least one of the well-known limits $lim_x to 0 fracsinxx$ or $lim_x to 0 fraccosx - 1x$ to come into use. They frequently did in my attempts but did not go anywhere. I have also been experimented substitutions but in vain.



How can this be solved without using L'Hospital's Rule?










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marked as duplicate by Jyrki Lahtonen, Nosrati, Lord Shark the Unknown, user91500, Jendrik Stelzner Sep 7 at 10:31


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.














  • Someone did downvote all answers, but s/he forgot to do downvote the question $;)$.
    – Nosrati
    Sep 7 at 5:15











  • @Nosrati: I am OP and I did not mark a single answer down, so please be more hesitant to pass judgement
    – hatinacat2000
    Sep 7 at 17:20










  • I didn't say you are downwoter! I said that for fun dear. Anyway I did delete my answer. $:)$ @hanti
    – Nosrati
    Sep 7 at 20:04















up vote
1
down vote

favorite













This question already has an answer here:



  • Limit as $xto 2$ of $fraccos(frac pi x)x-2 $

    5 answers



$$lim_x to 2 fraccosleft(fracpixright)x-2$$



This limit is supposed be found without L'Hospital's Rule, but I have not been able to get close to the answer using conjugates, squares, Pythagorean Identity, half angle formulas or Squeeze Theorem. For each attempt, I expected at least one of the well-known limits $lim_x to 0 fracsinxx$ or $lim_x to 0 fraccosx - 1x$ to come into use. They frequently did in my attempts but did not go anywhere. I have also been experimented substitutions but in vain.



How can this be solved without using L'Hospital's Rule?










share|cite|improve this question















marked as duplicate by Jyrki Lahtonen, Nosrati, Lord Shark the Unknown, user91500, Jendrik Stelzner Sep 7 at 10:31


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.














  • Someone did downvote all answers, but s/he forgot to do downvote the question $;)$.
    – Nosrati
    Sep 7 at 5:15











  • @Nosrati: I am OP and I did not mark a single answer down, so please be more hesitant to pass judgement
    – hatinacat2000
    Sep 7 at 17:20










  • I didn't say you are downwoter! I said that for fun dear. Anyway I did delete my answer. $:)$ @hanti
    – Nosrati
    Sep 7 at 20:04













up vote
1
down vote

favorite









up vote
1
down vote

favorite












This question already has an answer here:



  • Limit as $xto 2$ of $fraccos(frac pi x)x-2 $

    5 answers



$$lim_x to 2 fraccosleft(fracpixright)x-2$$



This limit is supposed be found without L'Hospital's Rule, but I have not been able to get close to the answer using conjugates, squares, Pythagorean Identity, half angle formulas or Squeeze Theorem. For each attempt, I expected at least one of the well-known limits $lim_x to 0 fracsinxx$ or $lim_x to 0 fraccosx - 1x$ to come into use. They frequently did in my attempts but did not go anywhere. I have also been experimented substitutions but in vain.



How can this be solved without using L'Hospital's Rule?










share|cite|improve this question
















This question already has an answer here:



  • Limit as $xto 2$ of $fraccos(frac pi x)x-2 $

    5 answers



$$lim_x to 2 fraccosleft(fracpixright)x-2$$



This limit is supposed be found without L'Hospital's Rule, but I have not been able to get close to the answer using conjugates, squares, Pythagorean Identity, half angle formulas or Squeeze Theorem. For each attempt, I expected at least one of the well-known limits $lim_x to 0 fracsinxx$ or $lim_x to 0 fraccosx - 1x$ to come into use. They frequently did in my attempts but did not go anywhere. I have also been experimented substitutions but in vain.



How can this be solved without using L'Hospital's Rule?





This question already has an answer here:



  • Limit as $xto 2$ of $fraccos(frac pi x)x-2 $

    5 answers







limits-without-lhopital






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edited Sep 7 at 10:30









Jendrik Stelzner

7,69121137




7,69121137










asked Sep 6 at 18:22









hatinacat2000

294




294




marked as duplicate by Jyrki Lahtonen, Nosrati, Lord Shark the Unknown, user91500, Jendrik Stelzner Sep 7 at 10:31


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.






marked as duplicate by Jyrki Lahtonen, Nosrati, Lord Shark the Unknown, user91500, Jendrik Stelzner Sep 7 at 10:31


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.













  • Someone did downvote all answers, but s/he forgot to do downvote the question $;)$.
    – Nosrati
    Sep 7 at 5:15











  • @Nosrati: I am OP and I did not mark a single answer down, so please be more hesitant to pass judgement
    – hatinacat2000
    Sep 7 at 17:20










  • I didn't say you are downwoter! I said that for fun dear. Anyway I did delete my answer. $:)$ @hanti
    – Nosrati
    Sep 7 at 20:04

















  • Someone did downvote all answers, but s/he forgot to do downvote the question $;)$.
    – Nosrati
    Sep 7 at 5:15











  • @Nosrati: I am OP and I did not mark a single answer down, so please be more hesitant to pass judgement
    – hatinacat2000
    Sep 7 at 17:20










  • I didn't say you are downwoter! I said that for fun dear. Anyway I did delete my answer. $:)$ @hanti
    – Nosrati
    Sep 7 at 20:04
















Someone did downvote all answers, but s/he forgot to do downvote the question $;)$.
– Nosrati
Sep 7 at 5:15





Someone did downvote all answers, but s/he forgot to do downvote the question $;)$.
– Nosrati
Sep 7 at 5:15













@Nosrati: I am OP and I did not mark a single answer down, so please be more hesitant to pass judgement
– hatinacat2000
Sep 7 at 17:20




@Nosrati: I am OP and I did not mark a single answer down, so please be more hesitant to pass judgement
– hatinacat2000
Sep 7 at 17:20












I didn't say you are downwoter! I said that for fun dear. Anyway I did delete my answer. $:)$ @hanti
– Nosrati
Sep 7 at 20:04





I didn't say you are downwoter! I said that for fun dear. Anyway I did delete my answer. $:)$ @hanti
– Nosrati
Sep 7 at 20:04











4 Answers
4






active

oldest

votes

















up vote
7
down vote













By definition of derivative, if $f(x)=cosleft(fracpi xright)$, then$$lim_xto2fraccosleft(fracpi xright)x-2=f'(2)=fracpi4.$$






share|cite|improve this answer


















  • 3




    intuition is a gift+
    – mrs
    Sep 6 at 18:28










  • Answer should be $pi/4$.
    – StammeringMathematician
    Sep 6 at 18:31






  • 1




    @StammeringMathematician I've edited my answer. Thank you.
    – José Carlos Santos
    Sep 6 at 18:32










  • IMO, using this trick is a little cheating. There is very little difference between $(f(x)-f(x_0))/(x-x_0)to f'(x_0)$ and $(f(x)-f(x_0))/(x-x_0)to f'(x_0)/1$.
    – Yves Daoust
    Sep 6 at 19:05


















up vote
1
down vote













With $dfracpi x=dfracpi2-t$,



$$limlimits_x to 2 fraccosleft(dfracpixright)x-2=limlimits_t to 0fracpi-2t4fracsinleft(tright)t.$$



No need to say more.






share|cite|improve this answer




















  • Can you please show your work? I have tried your substitution but am not able to get this result. Also, how did you come up with this substitution?
    – hatinacat2000
    Sep 7 at 5:03











  • @hatinacat2000: this is elementary algebra. And seeing the shape of the expression ($0/0$ with a trigonometric function), I understood that it could be turned to $sin t/t$ by an appropriate transform.
    – Yves Daoust
    Sep 7 at 7:12











  • I can get $frac sin tt $, but not $(pi - 2t)/4$
    – hatinacat2000
    Sep 7 at 17:19






  • 1




    Edit: success after more substitutions
    – hatinacat2000
    Sep 7 at 17:50

















up vote
1
down vote













This expression corresponds with the definition of the derivative. Recall that $$f'(x) = limlimits_yto xfracf(y)-f(x)y-x$$ Using the fact that $cosleft(fracpi2right) = 0$, this expression turns out to be the derivative of $cos(fracpix)$ at $x = 2$.



EDIT: Sorry, used the wrong function.






share|cite|improve this answer





























    up vote
    1
    down vote













    As an alternative, without derivatives, we have that



    $$ fraccosleft(fracpixright)x-2
    = fracfracpi2-fracpixx-2fracsinleft(fracpi2-fracpixright)fracpi2-fracpix
    =fracpi(x-2)2x(x-2)fracsinleft(fracpi2-fracpixright)fracpi2-fracpix=$$



    $$=fracpi2xfracsinleft(fracpi2-fracpixright)fracpi2-fracpixto fracpi4cdot 1 =fracpi4$$






    share|cite|improve this answer





























      4 Answers
      4






      active

      oldest

      votes








      4 Answers
      4






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes








      up vote
      7
      down vote













      By definition of derivative, if $f(x)=cosleft(fracpi xright)$, then$$lim_xto2fraccosleft(fracpi xright)x-2=f'(2)=fracpi4.$$






      share|cite|improve this answer


















      • 3




        intuition is a gift+
        – mrs
        Sep 6 at 18:28










      • Answer should be $pi/4$.
        – StammeringMathematician
        Sep 6 at 18:31






      • 1




        @StammeringMathematician I've edited my answer. Thank you.
        – José Carlos Santos
        Sep 6 at 18:32










      • IMO, using this trick is a little cheating. There is very little difference between $(f(x)-f(x_0))/(x-x_0)to f'(x_0)$ and $(f(x)-f(x_0))/(x-x_0)to f'(x_0)/1$.
        – Yves Daoust
        Sep 6 at 19:05















      up vote
      7
      down vote













      By definition of derivative, if $f(x)=cosleft(fracpi xright)$, then$$lim_xto2fraccosleft(fracpi xright)x-2=f'(2)=fracpi4.$$






      share|cite|improve this answer


















      • 3




        intuition is a gift+
        – mrs
        Sep 6 at 18:28










      • Answer should be $pi/4$.
        – StammeringMathematician
        Sep 6 at 18:31






      • 1




        @StammeringMathematician I've edited my answer. Thank you.
        – José Carlos Santos
        Sep 6 at 18:32










      • IMO, using this trick is a little cheating. There is very little difference between $(f(x)-f(x_0))/(x-x_0)to f'(x_0)$ and $(f(x)-f(x_0))/(x-x_0)to f'(x_0)/1$.
        – Yves Daoust
        Sep 6 at 19:05













      up vote
      7
      down vote










      up vote
      7
      down vote









      By definition of derivative, if $f(x)=cosleft(fracpi xright)$, then$$lim_xto2fraccosleft(fracpi xright)x-2=f'(2)=fracpi4.$$






      share|cite|improve this answer














      By definition of derivative, if $f(x)=cosleft(fracpi xright)$, then$$lim_xto2fraccosleft(fracpi xright)x-2=f'(2)=fracpi4.$$







      share|cite|improve this answer














      share|cite|improve this answer



      share|cite|improve this answer








      edited Sep 6 at 18:32

























      answered Sep 6 at 18:26









      José Carlos Santos

      123k17101186




      123k17101186







      • 3




        intuition is a gift+
        – mrs
        Sep 6 at 18:28










      • Answer should be $pi/4$.
        – StammeringMathematician
        Sep 6 at 18:31






      • 1




        @StammeringMathematician I've edited my answer. Thank you.
        – José Carlos Santos
        Sep 6 at 18:32










      • IMO, using this trick is a little cheating. There is very little difference between $(f(x)-f(x_0))/(x-x_0)to f'(x_0)$ and $(f(x)-f(x_0))/(x-x_0)to f'(x_0)/1$.
        – Yves Daoust
        Sep 6 at 19:05













      • 3




        intuition is a gift+
        – mrs
        Sep 6 at 18:28










      • Answer should be $pi/4$.
        – StammeringMathematician
        Sep 6 at 18:31






      • 1




        @StammeringMathematician I've edited my answer. Thank you.
        – José Carlos Santos
        Sep 6 at 18:32










      • IMO, using this trick is a little cheating. There is very little difference between $(f(x)-f(x_0))/(x-x_0)to f'(x_0)$ and $(f(x)-f(x_0))/(x-x_0)to f'(x_0)/1$.
        – Yves Daoust
        Sep 6 at 19:05








      3




      3




      intuition is a gift+
      – mrs
      Sep 6 at 18:28




      intuition is a gift+
      – mrs
      Sep 6 at 18:28












      Answer should be $pi/4$.
      – StammeringMathematician
      Sep 6 at 18:31




      Answer should be $pi/4$.
      – StammeringMathematician
      Sep 6 at 18:31




      1




      1




      @StammeringMathematician I've edited my answer. Thank you.
      – José Carlos Santos
      Sep 6 at 18:32




      @StammeringMathematician I've edited my answer. Thank you.
      – José Carlos Santos
      Sep 6 at 18:32












      IMO, using this trick is a little cheating. There is very little difference between $(f(x)-f(x_0))/(x-x_0)to f'(x_0)$ and $(f(x)-f(x_0))/(x-x_0)to f'(x_0)/1$.
      – Yves Daoust
      Sep 6 at 19:05





      IMO, using this trick is a little cheating. There is very little difference between $(f(x)-f(x_0))/(x-x_0)to f'(x_0)$ and $(f(x)-f(x_0))/(x-x_0)to f'(x_0)/1$.
      – Yves Daoust
      Sep 6 at 19:05











      up vote
      1
      down vote













      With $dfracpi x=dfracpi2-t$,



      $$limlimits_x to 2 fraccosleft(dfracpixright)x-2=limlimits_t to 0fracpi-2t4fracsinleft(tright)t.$$



      No need to say more.






      share|cite|improve this answer




















      • Can you please show your work? I have tried your substitution but am not able to get this result. Also, how did you come up with this substitution?
        – hatinacat2000
        Sep 7 at 5:03











      • @hatinacat2000: this is elementary algebra. And seeing the shape of the expression ($0/0$ with a trigonometric function), I understood that it could be turned to $sin t/t$ by an appropriate transform.
        – Yves Daoust
        Sep 7 at 7:12











      • I can get $frac sin tt $, but not $(pi - 2t)/4$
        – hatinacat2000
        Sep 7 at 17:19






      • 1




        Edit: success after more substitutions
        – hatinacat2000
        Sep 7 at 17:50














      up vote
      1
      down vote













      With $dfracpi x=dfracpi2-t$,



      $$limlimits_x to 2 fraccosleft(dfracpixright)x-2=limlimits_t to 0fracpi-2t4fracsinleft(tright)t.$$



      No need to say more.






      share|cite|improve this answer




















      • Can you please show your work? I have tried your substitution but am not able to get this result. Also, how did you come up with this substitution?
        – hatinacat2000
        Sep 7 at 5:03











      • @hatinacat2000: this is elementary algebra. And seeing the shape of the expression ($0/0$ with a trigonometric function), I understood that it could be turned to $sin t/t$ by an appropriate transform.
        – Yves Daoust
        Sep 7 at 7:12











      • I can get $frac sin tt $, but not $(pi - 2t)/4$
        – hatinacat2000
        Sep 7 at 17:19






      • 1




        Edit: success after more substitutions
        – hatinacat2000
        Sep 7 at 17:50












      up vote
      1
      down vote










      up vote
      1
      down vote









      With $dfracpi x=dfracpi2-t$,



      $$limlimits_x to 2 fraccosleft(dfracpixright)x-2=limlimits_t to 0fracpi-2t4fracsinleft(tright)t.$$



      No need to say more.






      share|cite|improve this answer












      With $dfracpi x=dfracpi2-t$,



      $$limlimits_x to 2 fraccosleft(dfracpixright)x-2=limlimits_t to 0fracpi-2t4fracsinleft(tright)t.$$



      No need to say more.







      share|cite|improve this answer












      share|cite|improve this answer



      share|cite|improve this answer










      answered Sep 6 at 18:36









      Yves Daoust

      115k666209




      115k666209











      • Can you please show your work? I have tried your substitution but am not able to get this result. Also, how did you come up with this substitution?
        – hatinacat2000
        Sep 7 at 5:03











      • @hatinacat2000: this is elementary algebra. And seeing the shape of the expression ($0/0$ with a trigonometric function), I understood that it could be turned to $sin t/t$ by an appropriate transform.
        – Yves Daoust
        Sep 7 at 7:12











      • I can get $frac sin tt $, but not $(pi - 2t)/4$
        – hatinacat2000
        Sep 7 at 17:19






      • 1




        Edit: success after more substitutions
        – hatinacat2000
        Sep 7 at 17:50
















      • Can you please show your work? I have tried your substitution but am not able to get this result. Also, how did you come up with this substitution?
        – hatinacat2000
        Sep 7 at 5:03











      • @hatinacat2000: this is elementary algebra. And seeing the shape of the expression ($0/0$ with a trigonometric function), I understood that it could be turned to $sin t/t$ by an appropriate transform.
        – Yves Daoust
        Sep 7 at 7:12











      • I can get $frac sin tt $, but not $(pi - 2t)/4$
        – hatinacat2000
        Sep 7 at 17:19






      • 1




        Edit: success after more substitutions
        – hatinacat2000
        Sep 7 at 17:50















      Can you please show your work? I have tried your substitution but am not able to get this result. Also, how did you come up with this substitution?
      – hatinacat2000
      Sep 7 at 5:03





      Can you please show your work? I have tried your substitution but am not able to get this result. Also, how did you come up with this substitution?
      – hatinacat2000
      Sep 7 at 5:03













      @hatinacat2000: this is elementary algebra. And seeing the shape of the expression ($0/0$ with a trigonometric function), I understood that it could be turned to $sin t/t$ by an appropriate transform.
      – Yves Daoust
      Sep 7 at 7:12





      @hatinacat2000: this is elementary algebra. And seeing the shape of the expression ($0/0$ with a trigonometric function), I understood that it could be turned to $sin t/t$ by an appropriate transform.
      – Yves Daoust
      Sep 7 at 7:12













      I can get $frac sin tt $, but not $(pi - 2t)/4$
      – hatinacat2000
      Sep 7 at 17:19




      I can get $frac sin tt $, but not $(pi - 2t)/4$
      – hatinacat2000
      Sep 7 at 17:19




      1




      1




      Edit: success after more substitutions
      – hatinacat2000
      Sep 7 at 17:50




      Edit: success after more substitutions
      – hatinacat2000
      Sep 7 at 17:50










      up vote
      1
      down vote













      This expression corresponds with the definition of the derivative. Recall that $$f'(x) = limlimits_yto xfracf(y)-f(x)y-x$$ Using the fact that $cosleft(fracpi2right) = 0$, this expression turns out to be the derivative of $cos(fracpix)$ at $x = 2$.



      EDIT: Sorry, used the wrong function.






      share|cite|improve this answer


























        up vote
        1
        down vote













        This expression corresponds with the definition of the derivative. Recall that $$f'(x) = limlimits_yto xfracf(y)-f(x)y-x$$ Using the fact that $cosleft(fracpi2right) = 0$, this expression turns out to be the derivative of $cos(fracpix)$ at $x = 2$.



        EDIT: Sorry, used the wrong function.






        share|cite|improve this answer
























          up vote
          1
          down vote










          up vote
          1
          down vote









          This expression corresponds with the definition of the derivative. Recall that $$f'(x) = limlimits_yto xfracf(y)-f(x)y-x$$ Using the fact that $cosleft(fracpi2right) = 0$, this expression turns out to be the derivative of $cos(fracpix)$ at $x = 2$.



          EDIT: Sorry, used the wrong function.






          share|cite|improve this answer














          This expression corresponds with the definition of the derivative. Recall that $$f'(x) = limlimits_yto xfracf(y)-f(x)y-x$$ Using the fact that $cosleft(fracpi2right) = 0$, this expression turns out to be the derivative of $cos(fracpix)$ at $x = 2$.



          EDIT: Sorry, used the wrong function.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Sep 6 at 18:54









          Bernard

          112k635104




          112k635104










          answered Sep 6 at 18:25









          Josh

          1789




          1789




















              up vote
              1
              down vote













              As an alternative, without derivatives, we have that



              $$ fraccosleft(fracpixright)x-2
              = fracfracpi2-fracpixx-2fracsinleft(fracpi2-fracpixright)fracpi2-fracpix
              =fracpi(x-2)2x(x-2)fracsinleft(fracpi2-fracpixright)fracpi2-fracpix=$$



              $$=fracpi2xfracsinleft(fracpi2-fracpixright)fracpi2-fracpixto fracpi4cdot 1 =fracpi4$$






              share|cite|improve this answer


























                up vote
                1
                down vote













                As an alternative, without derivatives, we have that



                $$ fraccosleft(fracpixright)x-2
                = fracfracpi2-fracpixx-2fracsinleft(fracpi2-fracpixright)fracpi2-fracpix
                =fracpi(x-2)2x(x-2)fracsinleft(fracpi2-fracpixright)fracpi2-fracpix=$$



                $$=fracpi2xfracsinleft(fracpi2-fracpixright)fracpi2-fracpixto fracpi4cdot 1 =fracpi4$$






                share|cite|improve this answer
























                  up vote
                  1
                  down vote










                  up vote
                  1
                  down vote









                  As an alternative, without derivatives, we have that



                  $$ fraccosleft(fracpixright)x-2
                  = fracfracpi2-fracpixx-2fracsinleft(fracpi2-fracpixright)fracpi2-fracpix
                  =fracpi(x-2)2x(x-2)fracsinleft(fracpi2-fracpixright)fracpi2-fracpix=$$



                  $$=fracpi2xfracsinleft(fracpi2-fracpixright)fracpi2-fracpixto fracpi4cdot 1 =fracpi4$$






                  share|cite|improve this answer














                  As an alternative, without derivatives, we have that



                  $$ fraccosleft(fracpixright)x-2
                  = fracfracpi2-fracpixx-2fracsinleft(fracpi2-fracpixright)fracpi2-fracpix
                  =fracpi(x-2)2x(x-2)fracsinleft(fracpi2-fracpixright)fracpi2-fracpix=$$



                  $$=fracpi2xfracsinleft(fracpi2-fracpixright)fracpi2-fracpixto fracpi4cdot 1 =fracpi4$$







                  share|cite|improve this answer














                  share|cite|improve this answer



                  share|cite|improve this answer








                  edited Sep 7 at 0:06

























                  answered Sep 6 at 19:55









                  gimusi

                  73.9k73889




                  73.9k73889












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