How do I compute $lim_{x to a}(a-x)tanleft(frac{πx}{2a}right)$












1












$begingroup$


Evaluate



$$lim_{x to a}f(x)=(a-x)tanleft(frac{πx}{2a}right)$$



I tried changing the tangent into cotangent by writing it in the form of $cotleft(dfrac{π}{2}-dfrac{πx}{2a}right)$. Then I factored out $dfrac{π}{2}$. But this didn't lead me to anything rigid. If anyone in the community could help me out, I'd really appreciate it.










share|cite|improve this question











$endgroup$












  • $begingroup$
    Do you know that you can "accept" one of the given answers? Please take a few minutes for a tour: math.stackexchange.com/tour
    $endgroup$
    – Robert Z
    Dec 12 '18 at 16:25
















1












$begingroup$


Evaluate



$$lim_{x to a}f(x)=(a-x)tanleft(frac{πx}{2a}right)$$



I tried changing the tangent into cotangent by writing it in the form of $cotleft(dfrac{π}{2}-dfrac{πx}{2a}right)$. Then I factored out $dfrac{π}{2}$. But this didn't lead me to anything rigid. If anyone in the community could help me out, I'd really appreciate it.










share|cite|improve this question











$endgroup$












  • $begingroup$
    Do you know that you can "accept" one of the given answers? Please take a few minutes for a tour: math.stackexchange.com/tour
    $endgroup$
    – Robert Z
    Dec 12 '18 at 16:25














1












1








1





$begingroup$


Evaluate



$$lim_{x to a}f(x)=(a-x)tanleft(frac{πx}{2a}right)$$



I tried changing the tangent into cotangent by writing it in the form of $cotleft(dfrac{π}{2}-dfrac{πx}{2a}right)$. Then I factored out $dfrac{π}{2}$. But this didn't lead me to anything rigid. If anyone in the community could help me out, I'd really appreciate it.










share|cite|improve this question











$endgroup$




Evaluate



$$lim_{x to a}f(x)=(a-x)tanleft(frac{πx}{2a}right)$$



I tried changing the tangent into cotangent by writing it in the form of $cotleft(dfrac{π}{2}-dfrac{πx}{2a}right)$. Then I factored out $dfrac{π}{2}$. But this didn't lead me to anything rigid. If anyone in the community could help me out, I'd really appreciate it.







limits limits-without-lhopital






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 11 '18 at 18:54









Lorenzo B.

1,8602520




1,8602520










asked Dec 11 '18 at 15:53









S.NepS.Nep

776




776












  • $begingroup$
    Do you know that you can "accept" one of the given answers? Please take a few minutes for a tour: math.stackexchange.com/tour
    $endgroup$
    – Robert Z
    Dec 12 '18 at 16:25


















  • $begingroup$
    Do you know that you can "accept" one of the given answers? Please take a few minutes for a tour: math.stackexchange.com/tour
    $endgroup$
    – Robert Z
    Dec 12 '18 at 16:25
















$begingroup$
Do you know that you can "accept" one of the given answers? Please take a few minutes for a tour: math.stackexchange.com/tour
$endgroup$
– Robert Z
Dec 12 '18 at 16:25




$begingroup$
Do you know that you can "accept" one of the given answers? Please take a few minutes for a tour: math.stackexchange.com/tour
$endgroup$
– Robert Z
Dec 12 '18 at 16:25










4 Answers
4






active

oldest

votes


















0












$begingroup$

As an alternative:
$$
lim_{x to a}f(x)=lim_{xto a}left((a-x)tanleft(frac{πx}{2a}right)right)
$$



Substitute $t = x - a iff x = t + a$, so your limit becomes:
$$
lim_{t to 0}f(t)=lim_{tto 0}left((a-(t+a))tanleft(frac{π(t+a)}{2a}right)right) = \
lim_{tto 0}left((-t)tanleft(frac{pi t + pi a}{2a}right)right) = -lim_{tto 0}left(ttanleft(frac{pi t}{2a} + frac{pi}{2}right)right) = \
= lim_{tto 0}left(tcotleft(frac{pi t}{2a}right)right) = lim_{tto0}frac{t}{tanleft(frac{pi t}{2a}right)}
$$



Now by Taylor of $tan x$ as $xto 0$:
$$
tan x sim x \
$$



We get:
$$
lim_{tto 0}frac{t}{tanleft(frac{pi t}{2a}right)} = lim_{tto 0}frac{2ta}{pi t} = fbox{$displaystyle frac{2a}{pi}$}
$$






share|cite|improve this answer









$endgroup$





















    2












    $begingroup$

    It does lead to a solution, keep going:
    $$begin{align}lim_{x to a}(a-x)tanleft(dfrac{πx}{2a}right)&=lim_{x to a}(a-x)cotleft(dfrac{π}{2}left(1-frac xaright)right)=\
    &=lim_{x to a}(a-x)cdot frac{color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}=\
    &=lim_{x to a} color{red}{frac{dfrac{π}{2}left(1-frac xaright)}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}}cdot frac{2a}{pi}=\
    &=frac{2a}{pi},end{align}$$

    where it was used:
    $$lim_{xto a} color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}=1;\
    lim_{xto 0} color{red}{frac{x}{sin x}}=lim_{xto 0} frac{sin x}{x}=1.$$






    share|cite|improve this answer









    $endgroup$





















      1












      $begingroup$

      Let $a-x=y$



      For $ane0,$



      $$tandfrac{pi x}{2a}=tandfrac{pi(a-y)}{2a}=cotdfrac{pi y}{2a}$$



      Use $lim_{hto0}dfrac{sin h}h=1$






      share|cite|improve this answer









      $endgroup$





















        1












        $begingroup$

        $a not =0$.



        Set $y =(πx)/(2a).$



        Then $y rightarrow π/2.$



        $((2a)/π)(π/2-y)dfrac{sin y}{cos y}.$



        Recall $sin (π/2-y) =cos y.$



        Then



        $(2a/π)(dfrac {1}{dfrac{sin (π/2-y)}{π/2-y}})(sin y)$.



        Can you take the limit $y rightarrow π/2?$



        P.S. Recall $lim_{z rightarrow 0}dfrac{sin z}{z}=1$.






        share|cite|improve this answer











        $endgroup$













          Your Answer





          StackExchange.ifUsing("editor", function () {
          return StackExchange.using("mathjaxEditing", function () {
          StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
          StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
          });
          });
          }, "mathjax-editing");

          StackExchange.ready(function() {
          var channelOptions = {
          tags: "".split(" "),
          id: "69"
          };
          initTagRenderer("".split(" "), "".split(" "), channelOptions);

          StackExchange.using("externalEditor", function() {
          // Have to fire editor after snippets, if snippets enabled
          if (StackExchange.settings.snippets.snippetsEnabled) {
          StackExchange.using("snippets", function() {
          createEditor();
          });
          }
          else {
          createEditor();
          }
          });

          function createEditor() {
          StackExchange.prepareEditor({
          heartbeatType: 'answer',
          autoActivateHeartbeat: false,
          convertImagesToLinks: true,
          noModals: true,
          showLowRepImageUploadWarning: true,
          reputationToPostImages: 10,
          bindNavPrevention: true,
          postfix: "",
          imageUploader: {
          brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
          contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
          allowUrls: true
          },
          noCode: true, onDemand: true,
          discardSelector: ".discard-answer"
          ,immediatelyShowMarkdownHelp:true
          });


          }
          });














          draft saved

          draft discarded


















          StackExchange.ready(
          function () {
          StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3035428%2fhow-do-i-compute-lim-x-to-aa-x-tan-left-frac%25cf%2580x2a-right%23new-answer', 'question_page');
          }
          );

          Post as a guest















          Required, but never shown

























          4 Answers
          4






          active

          oldest

          votes








          4 Answers
          4






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          0












          $begingroup$

          As an alternative:
          $$
          lim_{x to a}f(x)=lim_{xto a}left((a-x)tanleft(frac{πx}{2a}right)right)
          $$



          Substitute $t = x - a iff x = t + a$, so your limit becomes:
          $$
          lim_{t to 0}f(t)=lim_{tto 0}left((a-(t+a))tanleft(frac{π(t+a)}{2a}right)right) = \
          lim_{tto 0}left((-t)tanleft(frac{pi t + pi a}{2a}right)right) = -lim_{tto 0}left(ttanleft(frac{pi t}{2a} + frac{pi}{2}right)right) = \
          = lim_{tto 0}left(tcotleft(frac{pi t}{2a}right)right) = lim_{tto0}frac{t}{tanleft(frac{pi t}{2a}right)}
          $$



          Now by Taylor of $tan x$ as $xto 0$:
          $$
          tan x sim x \
          $$



          We get:
          $$
          lim_{tto 0}frac{t}{tanleft(frac{pi t}{2a}right)} = lim_{tto 0}frac{2ta}{pi t} = fbox{$displaystyle frac{2a}{pi}$}
          $$






          share|cite|improve this answer









          $endgroup$


















            0












            $begingroup$

            As an alternative:
            $$
            lim_{x to a}f(x)=lim_{xto a}left((a-x)tanleft(frac{πx}{2a}right)right)
            $$



            Substitute $t = x - a iff x = t + a$, so your limit becomes:
            $$
            lim_{t to 0}f(t)=lim_{tto 0}left((a-(t+a))tanleft(frac{π(t+a)}{2a}right)right) = \
            lim_{tto 0}left((-t)tanleft(frac{pi t + pi a}{2a}right)right) = -lim_{tto 0}left(ttanleft(frac{pi t}{2a} + frac{pi}{2}right)right) = \
            = lim_{tto 0}left(tcotleft(frac{pi t}{2a}right)right) = lim_{tto0}frac{t}{tanleft(frac{pi t}{2a}right)}
            $$



            Now by Taylor of $tan x$ as $xto 0$:
            $$
            tan x sim x \
            $$



            We get:
            $$
            lim_{tto 0}frac{t}{tanleft(frac{pi t}{2a}right)} = lim_{tto 0}frac{2ta}{pi t} = fbox{$displaystyle frac{2a}{pi}$}
            $$






            share|cite|improve this answer









            $endgroup$
















              0












              0








              0





              $begingroup$

              As an alternative:
              $$
              lim_{x to a}f(x)=lim_{xto a}left((a-x)tanleft(frac{πx}{2a}right)right)
              $$



              Substitute $t = x - a iff x = t + a$, so your limit becomes:
              $$
              lim_{t to 0}f(t)=lim_{tto 0}left((a-(t+a))tanleft(frac{π(t+a)}{2a}right)right) = \
              lim_{tto 0}left((-t)tanleft(frac{pi t + pi a}{2a}right)right) = -lim_{tto 0}left(ttanleft(frac{pi t}{2a} + frac{pi}{2}right)right) = \
              = lim_{tto 0}left(tcotleft(frac{pi t}{2a}right)right) = lim_{tto0}frac{t}{tanleft(frac{pi t}{2a}right)}
              $$



              Now by Taylor of $tan x$ as $xto 0$:
              $$
              tan x sim x \
              $$



              We get:
              $$
              lim_{tto 0}frac{t}{tanleft(frac{pi t}{2a}right)} = lim_{tto 0}frac{2ta}{pi t} = fbox{$displaystyle frac{2a}{pi}$}
              $$






              share|cite|improve this answer









              $endgroup$



              As an alternative:
              $$
              lim_{x to a}f(x)=lim_{xto a}left((a-x)tanleft(frac{πx}{2a}right)right)
              $$



              Substitute $t = x - a iff x = t + a$, so your limit becomes:
              $$
              lim_{t to 0}f(t)=lim_{tto 0}left((a-(t+a))tanleft(frac{π(t+a)}{2a}right)right) = \
              lim_{tto 0}left((-t)tanleft(frac{pi t + pi a}{2a}right)right) = -lim_{tto 0}left(ttanleft(frac{pi t}{2a} + frac{pi}{2}right)right) = \
              = lim_{tto 0}left(tcotleft(frac{pi t}{2a}right)right) = lim_{tto0}frac{t}{tanleft(frac{pi t}{2a}right)}
              $$



              Now by Taylor of $tan x$ as $xto 0$:
              $$
              tan x sim x \
              $$



              We get:
              $$
              lim_{tto 0}frac{t}{tanleft(frac{pi t}{2a}right)} = lim_{tto 0}frac{2ta}{pi t} = fbox{$displaystyle frac{2a}{pi}$}
              $$







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered Dec 13 '18 at 10:38









              romanroman

              2,28421224




              2,28421224























                  2












                  $begingroup$

                  It does lead to a solution, keep going:
                  $$begin{align}lim_{x to a}(a-x)tanleft(dfrac{πx}{2a}right)&=lim_{x to a}(a-x)cotleft(dfrac{π}{2}left(1-frac xaright)right)=\
                  &=lim_{x to a}(a-x)cdot frac{color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}=\
                  &=lim_{x to a} color{red}{frac{dfrac{π}{2}left(1-frac xaright)}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}}cdot frac{2a}{pi}=\
                  &=frac{2a}{pi},end{align}$$

                  where it was used:
                  $$lim_{xto a} color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}=1;\
                  lim_{xto 0} color{red}{frac{x}{sin x}}=lim_{xto 0} frac{sin x}{x}=1.$$






                  share|cite|improve this answer









                  $endgroup$


















                    2












                    $begingroup$

                    It does lead to a solution, keep going:
                    $$begin{align}lim_{x to a}(a-x)tanleft(dfrac{πx}{2a}right)&=lim_{x to a}(a-x)cotleft(dfrac{π}{2}left(1-frac xaright)right)=\
                    &=lim_{x to a}(a-x)cdot frac{color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}=\
                    &=lim_{x to a} color{red}{frac{dfrac{π}{2}left(1-frac xaright)}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}}cdot frac{2a}{pi}=\
                    &=frac{2a}{pi},end{align}$$

                    where it was used:
                    $$lim_{xto a} color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}=1;\
                    lim_{xto 0} color{red}{frac{x}{sin x}}=lim_{xto 0} frac{sin x}{x}=1.$$






                    share|cite|improve this answer









                    $endgroup$
















                      2












                      2








                      2





                      $begingroup$

                      It does lead to a solution, keep going:
                      $$begin{align}lim_{x to a}(a-x)tanleft(dfrac{πx}{2a}right)&=lim_{x to a}(a-x)cotleft(dfrac{π}{2}left(1-frac xaright)right)=\
                      &=lim_{x to a}(a-x)cdot frac{color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}=\
                      &=lim_{x to a} color{red}{frac{dfrac{π}{2}left(1-frac xaright)}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}}cdot frac{2a}{pi}=\
                      &=frac{2a}{pi},end{align}$$

                      where it was used:
                      $$lim_{xto a} color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}=1;\
                      lim_{xto 0} color{red}{frac{x}{sin x}}=lim_{xto 0} frac{sin x}{x}=1.$$






                      share|cite|improve this answer









                      $endgroup$



                      It does lead to a solution, keep going:
                      $$begin{align}lim_{x to a}(a-x)tanleft(dfrac{πx}{2a}right)&=lim_{x to a}(a-x)cotleft(dfrac{π}{2}left(1-frac xaright)right)=\
                      &=lim_{x to a}(a-x)cdot frac{color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}=\
                      &=lim_{x to a} color{red}{frac{dfrac{π}{2}left(1-frac xaright)}{sinleft(dfrac{π}{2}left(1-frac xaright)right)}}cdot frac{2a}{pi}=\
                      &=frac{2a}{pi},end{align}$$

                      where it was used:
                      $$lim_{xto a} color{blue}{cosleft(dfrac{π}{2}left(1-frac xaright)right)}=1;\
                      lim_{xto 0} color{red}{frac{x}{sin x}}=lim_{xto 0} frac{sin x}{x}=1.$$







                      share|cite|improve this answer












                      share|cite|improve this answer



                      share|cite|improve this answer










                      answered Dec 11 '18 at 18:26









                      farruhotafarruhota

                      20.5k2739




                      20.5k2739























                          1












                          $begingroup$

                          Let $a-x=y$



                          For $ane0,$



                          $$tandfrac{pi x}{2a}=tandfrac{pi(a-y)}{2a}=cotdfrac{pi y}{2a}$$



                          Use $lim_{hto0}dfrac{sin h}h=1$






                          share|cite|improve this answer









                          $endgroup$


















                            1












                            $begingroup$

                            Let $a-x=y$



                            For $ane0,$



                            $$tandfrac{pi x}{2a}=tandfrac{pi(a-y)}{2a}=cotdfrac{pi y}{2a}$$



                            Use $lim_{hto0}dfrac{sin h}h=1$






                            share|cite|improve this answer









                            $endgroup$
















                              1












                              1








                              1





                              $begingroup$

                              Let $a-x=y$



                              For $ane0,$



                              $$tandfrac{pi x}{2a}=tandfrac{pi(a-y)}{2a}=cotdfrac{pi y}{2a}$$



                              Use $lim_{hto0}dfrac{sin h}h=1$






                              share|cite|improve this answer









                              $endgroup$



                              Let $a-x=y$



                              For $ane0,$



                              $$tandfrac{pi x}{2a}=tandfrac{pi(a-y)}{2a}=cotdfrac{pi y}{2a}$$



                              Use $lim_{hto0}dfrac{sin h}h=1$







                              share|cite|improve this answer












                              share|cite|improve this answer



                              share|cite|improve this answer










                              answered Dec 11 '18 at 15:58









                              lab bhattacharjeelab bhattacharjee

                              226k15157275




                              226k15157275























                                  1












                                  $begingroup$

                                  $a not =0$.



                                  Set $y =(πx)/(2a).$



                                  Then $y rightarrow π/2.$



                                  $((2a)/π)(π/2-y)dfrac{sin y}{cos y}.$



                                  Recall $sin (π/2-y) =cos y.$



                                  Then



                                  $(2a/π)(dfrac {1}{dfrac{sin (π/2-y)}{π/2-y}})(sin y)$.



                                  Can you take the limit $y rightarrow π/2?$



                                  P.S. Recall $lim_{z rightarrow 0}dfrac{sin z}{z}=1$.






                                  share|cite|improve this answer











                                  $endgroup$


















                                    1












                                    $begingroup$

                                    $a not =0$.



                                    Set $y =(πx)/(2a).$



                                    Then $y rightarrow π/2.$



                                    $((2a)/π)(π/2-y)dfrac{sin y}{cos y}.$



                                    Recall $sin (π/2-y) =cos y.$



                                    Then



                                    $(2a/π)(dfrac {1}{dfrac{sin (π/2-y)}{π/2-y}})(sin y)$.



                                    Can you take the limit $y rightarrow π/2?$



                                    P.S. Recall $lim_{z rightarrow 0}dfrac{sin z}{z}=1$.






                                    share|cite|improve this answer











                                    $endgroup$
















                                      1












                                      1








                                      1





                                      $begingroup$

                                      $a not =0$.



                                      Set $y =(πx)/(2a).$



                                      Then $y rightarrow π/2.$



                                      $((2a)/π)(π/2-y)dfrac{sin y}{cos y}.$



                                      Recall $sin (π/2-y) =cos y.$



                                      Then



                                      $(2a/π)(dfrac {1}{dfrac{sin (π/2-y)}{π/2-y}})(sin y)$.



                                      Can you take the limit $y rightarrow π/2?$



                                      P.S. Recall $lim_{z rightarrow 0}dfrac{sin z}{z}=1$.






                                      share|cite|improve this answer











                                      $endgroup$



                                      $a not =0$.



                                      Set $y =(πx)/(2a).$



                                      Then $y rightarrow π/2.$



                                      $((2a)/π)(π/2-y)dfrac{sin y}{cos y}.$



                                      Recall $sin (π/2-y) =cos y.$



                                      Then



                                      $(2a/π)(dfrac {1}{dfrac{sin (π/2-y)}{π/2-y}})(sin y)$.



                                      Can you take the limit $y rightarrow π/2?$



                                      P.S. Recall $lim_{z rightarrow 0}dfrac{sin z}{z}=1$.







                                      share|cite|improve this answer














                                      share|cite|improve this answer



                                      share|cite|improve this answer








                                      edited Dec 13 '18 at 8:49

























                                      answered Dec 11 '18 at 16:25









                                      Peter SzilasPeter Szilas

                                      11.4k2822




                                      11.4k2822






























                                          draft saved

                                          draft discarded




















































                                          Thanks for contributing an answer to Mathematics Stack Exchange!


                                          • Please be sure to answer the question. Provide details and share your research!

                                          But avoid



                                          • Asking for help, clarification, or responding to other answers.

                                          • Making statements based on opinion; back them up with references or personal experience.


                                          Use MathJax to format equations. MathJax reference.


                                          To learn more, see our tips on writing great answers.




                                          draft saved


                                          draft discarded














                                          StackExchange.ready(
                                          function () {
                                          StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3035428%2fhow-do-i-compute-lim-x-to-aa-x-tan-left-frac%25cf%2580x2a-right%23new-answer', 'question_page');
                                          }
                                          );

                                          Post as a guest















                                          Required, but never shown





















































                                          Required, but never shown














                                          Required, but never shown












                                          Required, but never shown







                                          Required, but never shown

































                                          Required, but never shown














                                          Required, but never shown












                                          Required, but never shown







                                          Required, but never shown







                                          Popular posts from this blog

                                          Plaza Victoria

                                          Puebla de Zaragoza

                                          Musa