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.










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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.










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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






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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












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4 Answers
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$begingroup$

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






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






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    $endgroup$





















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






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          4 Answers
          4






          active

          oldest

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          4 Answers
          4






          active

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          active

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          active

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






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







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







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                      share|cite|improve this answer



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                      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






























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