Compute the limit:












0












$begingroup$


$$ lim_{x geq 0}{frac{1-cos^3(x)}{xsin(2x)}}$$
I tried:
$$lim_{x geq 0}{{frac{1-cos^2(x)*cos(x)}{2*x*sin^2(x)cos^2(x)}}}$$
$$lim_{x geq 0}{frac{cos^2x+sin^2x-cos^2(x)cos(x)}{2*x*sin^2x*cos^2(x)}}$$
$$lim_{x geq 0}{frac{cos^2(x)+sin^2(x)-(1-sin^2(x))cos(x)}{2*x*sin^2(x)*cos^2(x)}}$$
$$lim_{x geq 0}{frac{cos^2(x)+sin^2(x)-cos(x)+sin^2(x)cos(x)}{2*x*sin^2(x)*cos^2(x)}}$$
Somewhere I over-complicated and I don't know how to continue or the other route that I should take...










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$endgroup$

















    0












    $begingroup$


    $$ lim_{x geq 0}{frac{1-cos^3(x)}{xsin(2x)}}$$
    I tried:
    $$lim_{x geq 0}{{frac{1-cos^2(x)*cos(x)}{2*x*sin^2(x)cos^2(x)}}}$$
    $$lim_{x geq 0}{frac{cos^2x+sin^2x-cos^2(x)cos(x)}{2*x*sin^2x*cos^2(x)}}$$
    $$lim_{x geq 0}{frac{cos^2(x)+sin^2(x)-(1-sin^2(x))cos(x)}{2*x*sin^2(x)*cos^2(x)}}$$
    $$lim_{x geq 0}{frac{cos^2(x)+sin^2(x)-cos(x)+sin^2(x)cos(x)}{2*x*sin^2(x)*cos^2(x)}}$$
    Somewhere I over-complicated and I don't know how to continue or the other route that I should take...










    share|cite|improve this question











    $endgroup$















      0












      0








      0





      $begingroup$


      $$ lim_{x geq 0}{frac{1-cos^3(x)}{xsin(2x)}}$$
      I tried:
      $$lim_{x geq 0}{{frac{1-cos^2(x)*cos(x)}{2*x*sin^2(x)cos^2(x)}}}$$
      $$lim_{x geq 0}{frac{cos^2x+sin^2x-cos^2(x)cos(x)}{2*x*sin^2x*cos^2(x)}}$$
      $$lim_{x geq 0}{frac{cos^2(x)+sin^2(x)-(1-sin^2(x))cos(x)}{2*x*sin^2(x)*cos^2(x)}}$$
      $$lim_{x geq 0}{frac{cos^2(x)+sin^2(x)-cos(x)+sin^2(x)cos(x)}{2*x*sin^2(x)*cos^2(x)}}$$
      Somewhere I over-complicated and I don't know how to continue or the other route that I should take...










      share|cite|improve this question











      $endgroup$




      $$ lim_{x geq 0}{frac{1-cos^3(x)}{xsin(2x)}}$$
      I tried:
      $$lim_{x geq 0}{{frac{1-cos^2(x)*cos(x)}{2*x*sin^2(x)cos^2(x)}}}$$
      $$lim_{x geq 0}{frac{cos^2x+sin^2x-cos^2(x)cos(x)}{2*x*sin^2x*cos^2(x)}}$$
      $$lim_{x geq 0}{frac{cos^2(x)+sin^2(x)-(1-sin^2(x))cos(x)}{2*x*sin^2(x)*cos^2(x)}}$$
      $$lim_{x geq 0}{frac{cos^2(x)+sin^2(x)-cos(x)+sin^2(x)cos(x)}{2*x*sin^2(x)*cos^2(x)}}$$
      Somewhere I over-complicated and I don't know how to continue or the other route that I should take...







      limits trigonometry






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      share|cite|improve this question













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      share|cite|improve this question








      edited Dec 9 '18 at 0:48









      Key Flex

      8,61861233




      8,61861233










      asked Dec 8 '18 at 16:27









      Bili DebiliBili Debili

      1428




      1428






















          5 Answers
          5






          active

          oldest

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          2












          $begingroup$

          $$lim_{xrightarrow0}dfrac{1-cos^3x}{xsin(2x)}$$
          Apply L'Hopital's Rule$$lim_{xrightarrow0}dfrac{1-cos^3x}{xsin(2x)}=lim_{xrightarrow0}dfrac{3cos^2xsin x}{sin(2x)+2xcos(2x)}$$
          Again apply L'Hopital's Rule
          $$lim_{xrightarrow0}dfrac{3(-sin(2x)sin x+cos^3x)}{4cos(2x)-4xsin(2x)}=dfrac34$$






          share|cite|improve this answer









          $endgroup$





















            1












            $begingroup$

            Hint: Write
            $$frac{(1-cos(x))(1+cos(x))(cos^2(x)+cos(x)+1)}{2xsin(x)cos(x)(1+cos(x))}$$






            share|cite|improve this answer









            $endgroup$





















              1












              $begingroup$

              begin{align}lim_{x to 0}frac{1-cos^3x}{x sin (2x)}&= lim_{x to 0}frac{1-left( 1-frac{x^2}2right)^3}{2x^2}\
              &= lim_{x to 0} frac{1-(1-frac{3x^2}2)}{2x^2} \
              &= lim_{x to 0} frac{frac32x^2}{2x^2}\
              &= frac34end{align}






              share|cite|improve this answer









              $endgroup$













              • $begingroup$
                What is the step in between the first computation, where you get rid of trigonometric identities
                $endgroup$
                – Bili Debili
                Dec 8 '18 at 16:44












              • $begingroup$
                taylor series, just drop the higher order terms.
                $endgroup$
                – Siong Thye Goh
                Dec 8 '18 at 16:47






              • 1




                $begingroup$
                I always suggest to include remainder terms (in little-o or big-O form) when using Taylor's expansion, especialy when we introduce to the method not expert student.
                $endgroup$
                – gimusi
                Dec 8 '18 at 17:00



















              1












              $begingroup$

              We have that



              $${frac{1-cos^3(x)}{xsin(2x)}}={frac{1-cos(x)}{x^2}}cdot(1+cos x+cos^2(x))cdotfrac12{frac{2x}{sin(2x)}}$$



              then refer to standard limits




              • $frac{1-cos(x)}{x^2}to frac12$

              • $frac{2x}{sin(2x)}to 1$


              and simply




              • $1+cos x+cos^2(x)to 3$






              share|cite|improve this answer











              $endgroup$





















                0












                $begingroup$

                Let's write the expression under limit as $$frac{1-cos ^3x}{1-cos x} cdotfrac{1-cos x} {x^2}cdotfrac{2x}{sin 2x}cdotfrac{1}{2}$$ The first fraction tends to $3$, the second one tends to $1/2$, and the third one tends to $1$ and therefore the desired limit is $3/4$.






                share|cite|improve this answer









                $endgroup$













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                  5 Answers
                  5






                  active

                  oldest

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                  5 Answers
                  5






                  active

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                  active

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                  2












                  $begingroup$

                  $$lim_{xrightarrow0}dfrac{1-cos^3x}{xsin(2x)}$$
                  Apply L'Hopital's Rule$$lim_{xrightarrow0}dfrac{1-cos^3x}{xsin(2x)}=lim_{xrightarrow0}dfrac{3cos^2xsin x}{sin(2x)+2xcos(2x)}$$
                  Again apply L'Hopital's Rule
                  $$lim_{xrightarrow0}dfrac{3(-sin(2x)sin x+cos^3x)}{4cos(2x)-4xsin(2x)}=dfrac34$$






                  share|cite|improve this answer









                  $endgroup$


















                    2












                    $begingroup$

                    $$lim_{xrightarrow0}dfrac{1-cos^3x}{xsin(2x)}$$
                    Apply L'Hopital's Rule$$lim_{xrightarrow0}dfrac{1-cos^3x}{xsin(2x)}=lim_{xrightarrow0}dfrac{3cos^2xsin x}{sin(2x)+2xcos(2x)}$$
                    Again apply L'Hopital's Rule
                    $$lim_{xrightarrow0}dfrac{3(-sin(2x)sin x+cos^3x)}{4cos(2x)-4xsin(2x)}=dfrac34$$






                    share|cite|improve this answer









                    $endgroup$
















                      2












                      2








                      2





                      $begingroup$

                      $$lim_{xrightarrow0}dfrac{1-cos^3x}{xsin(2x)}$$
                      Apply L'Hopital's Rule$$lim_{xrightarrow0}dfrac{1-cos^3x}{xsin(2x)}=lim_{xrightarrow0}dfrac{3cos^2xsin x}{sin(2x)+2xcos(2x)}$$
                      Again apply L'Hopital's Rule
                      $$lim_{xrightarrow0}dfrac{3(-sin(2x)sin x+cos^3x)}{4cos(2x)-4xsin(2x)}=dfrac34$$






                      share|cite|improve this answer









                      $endgroup$



                      $$lim_{xrightarrow0}dfrac{1-cos^3x}{xsin(2x)}$$
                      Apply L'Hopital's Rule$$lim_{xrightarrow0}dfrac{1-cos^3x}{xsin(2x)}=lim_{xrightarrow0}dfrac{3cos^2xsin x}{sin(2x)+2xcos(2x)}$$
                      Again apply L'Hopital's Rule
                      $$lim_{xrightarrow0}dfrac{3(-sin(2x)sin x+cos^3x)}{4cos(2x)-4xsin(2x)}=dfrac34$$







                      share|cite|improve this answer












                      share|cite|improve this answer



                      share|cite|improve this answer










                      answered Dec 8 '18 at 16:41









                      Key FlexKey Flex

                      8,61861233




                      8,61861233























                          1












                          $begingroup$

                          Hint: Write
                          $$frac{(1-cos(x))(1+cos(x))(cos^2(x)+cos(x)+1)}{2xsin(x)cos(x)(1+cos(x))}$$






                          share|cite|improve this answer









                          $endgroup$


















                            1












                            $begingroup$

                            Hint: Write
                            $$frac{(1-cos(x))(1+cos(x))(cos^2(x)+cos(x)+1)}{2xsin(x)cos(x)(1+cos(x))}$$






                            share|cite|improve this answer









                            $endgroup$
















                              1












                              1








                              1





                              $begingroup$

                              Hint: Write
                              $$frac{(1-cos(x))(1+cos(x))(cos^2(x)+cos(x)+1)}{2xsin(x)cos(x)(1+cos(x))}$$






                              share|cite|improve this answer









                              $endgroup$



                              Hint: Write
                              $$frac{(1-cos(x))(1+cos(x))(cos^2(x)+cos(x)+1)}{2xsin(x)cos(x)(1+cos(x))}$$







                              share|cite|improve this answer












                              share|cite|improve this answer



                              share|cite|improve this answer










                              answered Dec 8 '18 at 16:33









                              Dr. Sonnhard GraubnerDr. Sonnhard Graubner

                              77.8k42866




                              77.8k42866























                                  1












                                  $begingroup$

                                  begin{align}lim_{x to 0}frac{1-cos^3x}{x sin (2x)}&= lim_{x to 0}frac{1-left( 1-frac{x^2}2right)^3}{2x^2}\
                                  &= lim_{x to 0} frac{1-(1-frac{3x^2}2)}{2x^2} \
                                  &= lim_{x to 0} frac{frac32x^2}{2x^2}\
                                  &= frac34end{align}






                                  share|cite|improve this answer









                                  $endgroup$













                                  • $begingroup$
                                    What is the step in between the first computation, where you get rid of trigonometric identities
                                    $endgroup$
                                    – Bili Debili
                                    Dec 8 '18 at 16:44












                                  • $begingroup$
                                    taylor series, just drop the higher order terms.
                                    $endgroup$
                                    – Siong Thye Goh
                                    Dec 8 '18 at 16:47






                                  • 1




                                    $begingroup$
                                    I always suggest to include remainder terms (in little-o or big-O form) when using Taylor's expansion, especialy when we introduce to the method not expert student.
                                    $endgroup$
                                    – gimusi
                                    Dec 8 '18 at 17:00
















                                  1












                                  $begingroup$

                                  begin{align}lim_{x to 0}frac{1-cos^3x}{x sin (2x)}&= lim_{x to 0}frac{1-left( 1-frac{x^2}2right)^3}{2x^2}\
                                  &= lim_{x to 0} frac{1-(1-frac{3x^2}2)}{2x^2} \
                                  &= lim_{x to 0} frac{frac32x^2}{2x^2}\
                                  &= frac34end{align}






                                  share|cite|improve this answer









                                  $endgroup$













                                  • $begingroup$
                                    What is the step in between the first computation, where you get rid of trigonometric identities
                                    $endgroup$
                                    – Bili Debili
                                    Dec 8 '18 at 16:44












                                  • $begingroup$
                                    taylor series, just drop the higher order terms.
                                    $endgroup$
                                    – Siong Thye Goh
                                    Dec 8 '18 at 16:47






                                  • 1




                                    $begingroup$
                                    I always suggest to include remainder terms (in little-o or big-O form) when using Taylor's expansion, especialy when we introduce to the method not expert student.
                                    $endgroup$
                                    – gimusi
                                    Dec 8 '18 at 17:00














                                  1












                                  1








                                  1





                                  $begingroup$

                                  begin{align}lim_{x to 0}frac{1-cos^3x}{x sin (2x)}&= lim_{x to 0}frac{1-left( 1-frac{x^2}2right)^3}{2x^2}\
                                  &= lim_{x to 0} frac{1-(1-frac{3x^2}2)}{2x^2} \
                                  &= lim_{x to 0} frac{frac32x^2}{2x^2}\
                                  &= frac34end{align}






                                  share|cite|improve this answer









                                  $endgroup$



                                  begin{align}lim_{x to 0}frac{1-cos^3x}{x sin (2x)}&= lim_{x to 0}frac{1-left( 1-frac{x^2}2right)^3}{2x^2}\
                                  &= lim_{x to 0} frac{1-(1-frac{3x^2}2)}{2x^2} \
                                  &= lim_{x to 0} frac{frac32x^2}{2x^2}\
                                  &= frac34end{align}







                                  share|cite|improve this answer












                                  share|cite|improve this answer



                                  share|cite|improve this answer










                                  answered Dec 8 '18 at 16:37









                                  Siong Thye GohSiong Thye Goh

                                  103k1468119




                                  103k1468119












                                  • $begingroup$
                                    What is the step in between the first computation, where you get rid of trigonometric identities
                                    $endgroup$
                                    – Bili Debili
                                    Dec 8 '18 at 16:44












                                  • $begingroup$
                                    taylor series, just drop the higher order terms.
                                    $endgroup$
                                    – Siong Thye Goh
                                    Dec 8 '18 at 16:47






                                  • 1




                                    $begingroup$
                                    I always suggest to include remainder terms (in little-o or big-O form) when using Taylor's expansion, especialy when we introduce to the method not expert student.
                                    $endgroup$
                                    – gimusi
                                    Dec 8 '18 at 17:00


















                                  • $begingroup$
                                    What is the step in between the first computation, where you get rid of trigonometric identities
                                    $endgroup$
                                    – Bili Debili
                                    Dec 8 '18 at 16:44












                                  • $begingroup$
                                    taylor series, just drop the higher order terms.
                                    $endgroup$
                                    – Siong Thye Goh
                                    Dec 8 '18 at 16:47






                                  • 1




                                    $begingroup$
                                    I always suggest to include remainder terms (in little-o or big-O form) when using Taylor's expansion, especialy when we introduce to the method not expert student.
                                    $endgroup$
                                    – gimusi
                                    Dec 8 '18 at 17:00
















                                  $begingroup$
                                  What is the step in between the first computation, where you get rid of trigonometric identities
                                  $endgroup$
                                  – Bili Debili
                                  Dec 8 '18 at 16:44






                                  $begingroup$
                                  What is the step in between the first computation, where you get rid of trigonometric identities
                                  $endgroup$
                                  – Bili Debili
                                  Dec 8 '18 at 16:44














                                  $begingroup$
                                  taylor series, just drop the higher order terms.
                                  $endgroup$
                                  – Siong Thye Goh
                                  Dec 8 '18 at 16:47




                                  $begingroup$
                                  taylor series, just drop the higher order terms.
                                  $endgroup$
                                  – Siong Thye Goh
                                  Dec 8 '18 at 16:47




                                  1




                                  1




                                  $begingroup$
                                  I always suggest to include remainder terms (in little-o or big-O form) when using Taylor's expansion, especialy when we introduce to the method not expert student.
                                  $endgroup$
                                  – gimusi
                                  Dec 8 '18 at 17:00




                                  $begingroup$
                                  I always suggest to include remainder terms (in little-o or big-O form) when using Taylor's expansion, especialy when we introduce to the method not expert student.
                                  $endgroup$
                                  – gimusi
                                  Dec 8 '18 at 17:00











                                  1












                                  $begingroup$

                                  We have that



                                  $${frac{1-cos^3(x)}{xsin(2x)}}={frac{1-cos(x)}{x^2}}cdot(1+cos x+cos^2(x))cdotfrac12{frac{2x}{sin(2x)}}$$



                                  then refer to standard limits




                                  • $frac{1-cos(x)}{x^2}to frac12$

                                  • $frac{2x}{sin(2x)}to 1$


                                  and simply




                                  • $1+cos x+cos^2(x)to 3$






                                  share|cite|improve this answer











                                  $endgroup$


















                                    1












                                    $begingroup$

                                    We have that



                                    $${frac{1-cos^3(x)}{xsin(2x)}}={frac{1-cos(x)}{x^2}}cdot(1+cos x+cos^2(x))cdotfrac12{frac{2x}{sin(2x)}}$$



                                    then refer to standard limits




                                    • $frac{1-cos(x)}{x^2}to frac12$

                                    • $frac{2x}{sin(2x)}to 1$


                                    and simply




                                    • $1+cos x+cos^2(x)to 3$






                                    share|cite|improve this answer











                                    $endgroup$
















                                      1












                                      1








                                      1





                                      $begingroup$

                                      We have that



                                      $${frac{1-cos^3(x)}{xsin(2x)}}={frac{1-cos(x)}{x^2}}cdot(1+cos x+cos^2(x))cdotfrac12{frac{2x}{sin(2x)}}$$



                                      then refer to standard limits




                                      • $frac{1-cos(x)}{x^2}to frac12$

                                      • $frac{2x}{sin(2x)}to 1$


                                      and simply




                                      • $1+cos x+cos^2(x)to 3$






                                      share|cite|improve this answer











                                      $endgroup$



                                      We have that



                                      $${frac{1-cos^3(x)}{xsin(2x)}}={frac{1-cos(x)}{x^2}}cdot(1+cos x+cos^2(x))cdotfrac12{frac{2x}{sin(2x)}}$$



                                      then refer to standard limits




                                      • $frac{1-cos(x)}{x^2}to frac12$

                                      • $frac{2x}{sin(2x)}to 1$


                                      and simply




                                      • $1+cos x+cos^2(x)to 3$







                                      share|cite|improve this answer














                                      share|cite|improve this answer



                                      share|cite|improve this answer








                                      edited Dec 8 '18 at 16:59

























                                      answered Dec 8 '18 at 16:30









                                      gimusigimusi

                                      93k84594




                                      93k84594























                                          0












                                          $begingroup$

                                          Let's write the expression under limit as $$frac{1-cos ^3x}{1-cos x} cdotfrac{1-cos x} {x^2}cdotfrac{2x}{sin 2x}cdotfrac{1}{2}$$ The first fraction tends to $3$, the second one tends to $1/2$, and the third one tends to $1$ and therefore the desired limit is $3/4$.






                                          share|cite|improve this answer









                                          $endgroup$


















                                            0












                                            $begingroup$

                                            Let's write the expression under limit as $$frac{1-cos ^3x}{1-cos x} cdotfrac{1-cos x} {x^2}cdotfrac{2x}{sin 2x}cdotfrac{1}{2}$$ The first fraction tends to $3$, the second one tends to $1/2$, and the third one tends to $1$ and therefore the desired limit is $3/4$.






                                            share|cite|improve this answer









                                            $endgroup$
















                                              0












                                              0








                                              0





                                              $begingroup$

                                              Let's write the expression under limit as $$frac{1-cos ^3x}{1-cos x} cdotfrac{1-cos x} {x^2}cdotfrac{2x}{sin 2x}cdotfrac{1}{2}$$ The first fraction tends to $3$, the second one tends to $1/2$, and the third one tends to $1$ and therefore the desired limit is $3/4$.






                                              share|cite|improve this answer









                                              $endgroup$



                                              Let's write the expression under limit as $$frac{1-cos ^3x}{1-cos x} cdotfrac{1-cos x} {x^2}cdotfrac{2x}{sin 2x}cdotfrac{1}{2}$$ The first fraction tends to $3$, the second one tends to $1/2$, and the third one tends to $1$ and therefore the desired limit is $3/4$.







                                              share|cite|improve this answer












                                              share|cite|improve this answer



                                              share|cite|improve this answer










                                              answered Dec 9 '18 at 5:06









                                              Paramanand SinghParamanand Singh

                                              50.7k557168




                                              50.7k557168






























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