De Moivres Theorem












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This is the questions I got and I'm stuck as to where to proceed after these few steps
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my solution so far










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    $begingroup$


    This is the questions I got and I'm stuck as to where to proceed after these few steps
    the question



    my solution so far










    share|cite|improve this question









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      1



      $begingroup$


      This is the questions I got and I'm stuck as to where to proceed after these few steps
      the question



      my solution so far










      share|cite|improve this question









      $endgroup$




      This is the questions I got and I'm stuck as to where to proceed after these few steps
      the question



      my solution so far







      complex-numbers






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      asked Jan 24 at 2:29









      user122343user122343

      805




      805






















          4 Answers
          4






          active

          oldest

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          1












          $begingroup$

          For the second part since we want to solve the equation $$x^4-6x^2+1=0$$ in terms of $tan(frac{npi}{8})$ we assume that:
          $$frac{npi}{8}=theta$$
          Now $$4theta = frac{npi}{2}$$
          Therefore, $$tan(4theta) = tan(frac{npi}{2})$$
          RHS is always either $infty$ or $-infty$ so whatever expansion of $tan(4theta)$ is in the denominator that will obviously be $0$.
          Now since, denominator is $0$ or $$tan^4theta-6tan^2theta+1=0$$
          Now you assume that $$x=tantheta$$ and solve the biquadratic equation to get the answer. The equation will become:
          $$x^4-6x^2+1=0$$
          How to solve this equation?? Put $$x^2=t$$ and rewrite the biquadratic equation as a quadratic equation in $t$ giving
          $$t^2-6t+1=0$$. Solve for $t$ and then put $x^2=t$ to get the roots of x.
          Hope this helps ......






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thanks it did help
            $endgroup$
            – user122343
            Jan 25 at 10:26










          • $begingroup$
            I am happy to know that, welcome
            $endgroup$
            – SNEHIL SANYAL
            Jan 25 at 13:23



















          2












          $begingroup$

          For the first part of the question:
          Notice by DMT:



          $$(costheta +isintheta)^4=cos(4theta)+isin(4theta)$$
          Since $tan x=frac{sin x}{cos x}$, we have that: $$tan(4theta)=frac{Im((costheta+isintheta)^4)}{Re((costheta+isintheta)^4)}$$



          Use the binomial expansion to do the rest.






          share|cite|improve this answer









          $endgroup$





















            2












            $begingroup$

            For the second part, set $dfrac1{tan4theta }=0$



            $implies4theta=(2n+1)dfracpi2$ for some integer $n$



            $theta=dfrac{(2n+1)pi}8$ where $nequiv0,1,2,3pmod4$






            share|cite|improve this answer









            $endgroup$





















              1












              $begingroup$

              $$sin(4 theta) = 2 sin (2 theta) cos(2 theta)$$
              $$cos(4 theta) = cos^2(2 theta)-sin^2(2theta)$$
              $$sin(2theta) = 2sin(theta)cos(theta)$$
              $$cos(2theta) = cos^2(theta)-sin^2(theta)$$



              Note that all of these can be derived from de Moivre's theorem $(cos theta + j sin theta)^n = cos(n theta) + jsin(n theta)$.



              For example, when $n=2$,
              $$(cos theta + j sin theta)^2 = cos^2(theta) - sin^2(theta) + j2 sin theta cos theta = cos(2 theta) + jsin(2 theta)$$
              implying
              $$sin(2theta) = 2sin(theta)cos(theta)$$
              $$cos(2theta) = cos^2(theta)-sin^2(theta)$$



              You can similarly derive the result for $n=4$.



              Anyway, now you can compute
              $$tan(4 theta) = {sin(4 theta) over cos(4 theta)} = {4 sin theta cos theta ; (cos^2 theta - sin^2 theta) over (cos^2 theta - sin^2 theta)^2 - 4sin^2 theta cos^2 theta}$$



              If you divide the numerator and denominator by $cos^4 theta$, this simplifies to



              $$tan(4 theta) = {4 tan theta - 4 tan^3 theta over 1 - 6 tan^2 theta + tan^4 theta}$$



              Now, note that when $4 theta = n pi/2$, $tan 4 theta = infty$, i.e. $1 - 6 tan^2 theta + tan^4 theta = 0$. This is exactly the equation you want to solve (with $x = tan theta$), so you get $x = tan (n pi/8)$.






              share|cite|improve this answer









              $endgroup$













                Your Answer





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                4 Answers
                4






                active

                oldest

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                4 Answers
                4






                active

                oldest

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                active

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                active

                oldest

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                1












                $begingroup$

                For the second part since we want to solve the equation $$x^4-6x^2+1=0$$ in terms of $tan(frac{npi}{8})$ we assume that:
                $$frac{npi}{8}=theta$$
                Now $$4theta = frac{npi}{2}$$
                Therefore, $$tan(4theta) = tan(frac{npi}{2})$$
                RHS is always either $infty$ or $-infty$ so whatever expansion of $tan(4theta)$ is in the denominator that will obviously be $0$.
                Now since, denominator is $0$ or $$tan^4theta-6tan^2theta+1=0$$
                Now you assume that $$x=tantheta$$ and solve the biquadratic equation to get the answer. The equation will become:
                $$x^4-6x^2+1=0$$
                How to solve this equation?? Put $$x^2=t$$ and rewrite the biquadratic equation as a quadratic equation in $t$ giving
                $$t^2-6t+1=0$$. Solve for $t$ and then put $x^2=t$ to get the roots of x.
                Hope this helps ......






                share|cite|improve this answer









                $endgroup$













                • $begingroup$
                  Thanks it did help
                  $endgroup$
                  – user122343
                  Jan 25 at 10:26










                • $begingroup$
                  I am happy to know that, welcome
                  $endgroup$
                  – SNEHIL SANYAL
                  Jan 25 at 13:23
















                1












                $begingroup$

                For the second part since we want to solve the equation $$x^4-6x^2+1=0$$ in terms of $tan(frac{npi}{8})$ we assume that:
                $$frac{npi}{8}=theta$$
                Now $$4theta = frac{npi}{2}$$
                Therefore, $$tan(4theta) = tan(frac{npi}{2})$$
                RHS is always either $infty$ or $-infty$ so whatever expansion of $tan(4theta)$ is in the denominator that will obviously be $0$.
                Now since, denominator is $0$ or $$tan^4theta-6tan^2theta+1=0$$
                Now you assume that $$x=tantheta$$ and solve the biquadratic equation to get the answer. The equation will become:
                $$x^4-6x^2+1=0$$
                How to solve this equation?? Put $$x^2=t$$ and rewrite the biquadratic equation as a quadratic equation in $t$ giving
                $$t^2-6t+1=0$$. Solve for $t$ and then put $x^2=t$ to get the roots of x.
                Hope this helps ......






                share|cite|improve this answer









                $endgroup$













                • $begingroup$
                  Thanks it did help
                  $endgroup$
                  – user122343
                  Jan 25 at 10:26










                • $begingroup$
                  I am happy to know that, welcome
                  $endgroup$
                  – SNEHIL SANYAL
                  Jan 25 at 13:23














                1












                1








                1





                $begingroup$

                For the second part since we want to solve the equation $$x^4-6x^2+1=0$$ in terms of $tan(frac{npi}{8})$ we assume that:
                $$frac{npi}{8}=theta$$
                Now $$4theta = frac{npi}{2}$$
                Therefore, $$tan(4theta) = tan(frac{npi}{2})$$
                RHS is always either $infty$ or $-infty$ so whatever expansion of $tan(4theta)$ is in the denominator that will obviously be $0$.
                Now since, denominator is $0$ or $$tan^4theta-6tan^2theta+1=0$$
                Now you assume that $$x=tantheta$$ and solve the biquadratic equation to get the answer. The equation will become:
                $$x^4-6x^2+1=0$$
                How to solve this equation?? Put $$x^2=t$$ and rewrite the biquadratic equation as a quadratic equation in $t$ giving
                $$t^2-6t+1=0$$. Solve for $t$ and then put $x^2=t$ to get the roots of x.
                Hope this helps ......






                share|cite|improve this answer









                $endgroup$



                For the second part since we want to solve the equation $$x^4-6x^2+1=0$$ in terms of $tan(frac{npi}{8})$ we assume that:
                $$frac{npi}{8}=theta$$
                Now $$4theta = frac{npi}{2}$$
                Therefore, $$tan(4theta) = tan(frac{npi}{2})$$
                RHS is always either $infty$ or $-infty$ so whatever expansion of $tan(4theta)$ is in the denominator that will obviously be $0$.
                Now since, denominator is $0$ or $$tan^4theta-6tan^2theta+1=0$$
                Now you assume that $$x=tantheta$$ and solve the biquadratic equation to get the answer. The equation will become:
                $$x^4-6x^2+1=0$$
                How to solve this equation?? Put $$x^2=t$$ and rewrite the biquadratic equation as a quadratic equation in $t$ giving
                $$t^2-6t+1=0$$. Solve for $t$ and then put $x^2=t$ to get the roots of x.
                Hope this helps ......







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Jan 24 at 4:02









                SNEHIL SANYALSNEHIL SANYAL

                614110




                614110












                • $begingroup$
                  Thanks it did help
                  $endgroup$
                  – user122343
                  Jan 25 at 10:26










                • $begingroup$
                  I am happy to know that, welcome
                  $endgroup$
                  – SNEHIL SANYAL
                  Jan 25 at 13:23


















                • $begingroup$
                  Thanks it did help
                  $endgroup$
                  – user122343
                  Jan 25 at 10:26










                • $begingroup$
                  I am happy to know that, welcome
                  $endgroup$
                  – SNEHIL SANYAL
                  Jan 25 at 13:23
















                $begingroup$
                Thanks it did help
                $endgroup$
                – user122343
                Jan 25 at 10:26




                $begingroup$
                Thanks it did help
                $endgroup$
                – user122343
                Jan 25 at 10:26












                $begingroup$
                I am happy to know that, welcome
                $endgroup$
                – SNEHIL SANYAL
                Jan 25 at 13:23




                $begingroup$
                I am happy to know that, welcome
                $endgroup$
                – SNEHIL SANYAL
                Jan 25 at 13:23











                2












                $begingroup$

                For the first part of the question:
                Notice by DMT:



                $$(costheta +isintheta)^4=cos(4theta)+isin(4theta)$$
                Since $tan x=frac{sin x}{cos x}$, we have that: $$tan(4theta)=frac{Im((costheta+isintheta)^4)}{Re((costheta+isintheta)^4)}$$



                Use the binomial expansion to do the rest.






                share|cite|improve this answer









                $endgroup$


















                  2












                  $begingroup$

                  For the first part of the question:
                  Notice by DMT:



                  $$(costheta +isintheta)^4=cos(4theta)+isin(4theta)$$
                  Since $tan x=frac{sin x}{cos x}$, we have that: $$tan(4theta)=frac{Im((costheta+isintheta)^4)}{Re((costheta+isintheta)^4)}$$



                  Use the binomial expansion to do the rest.






                  share|cite|improve this answer









                  $endgroup$
















                    2












                    2








                    2





                    $begingroup$

                    For the first part of the question:
                    Notice by DMT:



                    $$(costheta +isintheta)^4=cos(4theta)+isin(4theta)$$
                    Since $tan x=frac{sin x}{cos x}$, we have that: $$tan(4theta)=frac{Im((costheta+isintheta)^4)}{Re((costheta+isintheta)^4)}$$



                    Use the binomial expansion to do the rest.






                    share|cite|improve this answer









                    $endgroup$



                    For the first part of the question:
                    Notice by DMT:



                    $$(costheta +isintheta)^4=cos(4theta)+isin(4theta)$$
                    Since $tan x=frac{sin x}{cos x}$, we have that: $$tan(4theta)=frac{Im((costheta+isintheta)^4)}{Re((costheta+isintheta)^4)}$$



                    Use the binomial expansion to do the rest.







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Jan 24 at 3:36









                    Rhys HughesRhys Hughes

                    6,9571530




                    6,9571530























                        2












                        $begingroup$

                        For the second part, set $dfrac1{tan4theta }=0$



                        $implies4theta=(2n+1)dfracpi2$ for some integer $n$



                        $theta=dfrac{(2n+1)pi}8$ where $nequiv0,1,2,3pmod4$






                        share|cite|improve this answer









                        $endgroup$


















                          2












                          $begingroup$

                          For the second part, set $dfrac1{tan4theta }=0$



                          $implies4theta=(2n+1)dfracpi2$ for some integer $n$



                          $theta=dfrac{(2n+1)pi}8$ where $nequiv0,1,2,3pmod4$






                          share|cite|improve this answer









                          $endgroup$
















                            2












                            2








                            2





                            $begingroup$

                            For the second part, set $dfrac1{tan4theta }=0$



                            $implies4theta=(2n+1)dfracpi2$ for some integer $n$



                            $theta=dfrac{(2n+1)pi}8$ where $nequiv0,1,2,3pmod4$






                            share|cite|improve this answer









                            $endgroup$



                            For the second part, set $dfrac1{tan4theta }=0$



                            $implies4theta=(2n+1)dfracpi2$ for some integer $n$



                            $theta=dfrac{(2n+1)pi}8$ where $nequiv0,1,2,3pmod4$







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Jan 24 at 3:55









                            lab bhattacharjeelab bhattacharjee

                            226k15157275




                            226k15157275























                                1












                                $begingroup$

                                $$sin(4 theta) = 2 sin (2 theta) cos(2 theta)$$
                                $$cos(4 theta) = cos^2(2 theta)-sin^2(2theta)$$
                                $$sin(2theta) = 2sin(theta)cos(theta)$$
                                $$cos(2theta) = cos^2(theta)-sin^2(theta)$$



                                Note that all of these can be derived from de Moivre's theorem $(cos theta + j sin theta)^n = cos(n theta) + jsin(n theta)$.



                                For example, when $n=2$,
                                $$(cos theta + j sin theta)^2 = cos^2(theta) - sin^2(theta) + j2 sin theta cos theta = cos(2 theta) + jsin(2 theta)$$
                                implying
                                $$sin(2theta) = 2sin(theta)cos(theta)$$
                                $$cos(2theta) = cos^2(theta)-sin^2(theta)$$



                                You can similarly derive the result for $n=4$.



                                Anyway, now you can compute
                                $$tan(4 theta) = {sin(4 theta) over cos(4 theta)} = {4 sin theta cos theta ; (cos^2 theta - sin^2 theta) over (cos^2 theta - sin^2 theta)^2 - 4sin^2 theta cos^2 theta}$$



                                If you divide the numerator and denominator by $cos^4 theta$, this simplifies to



                                $$tan(4 theta) = {4 tan theta - 4 tan^3 theta over 1 - 6 tan^2 theta + tan^4 theta}$$



                                Now, note that when $4 theta = n pi/2$, $tan 4 theta = infty$, i.e. $1 - 6 tan^2 theta + tan^4 theta = 0$. This is exactly the equation you want to solve (with $x = tan theta$), so you get $x = tan (n pi/8)$.






                                share|cite|improve this answer









                                $endgroup$


















                                  1












                                  $begingroup$

                                  $$sin(4 theta) = 2 sin (2 theta) cos(2 theta)$$
                                  $$cos(4 theta) = cos^2(2 theta)-sin^2(2theta)$$
                                  $$sin(2theta) = 2sin(theta)cos(theta)$$
                                  $$cos(2theta) = cos^2(theta)-sin^2(theta)$$



                                  Note that all of these can be derived from de Moivre's theorem $(cos theta + j sin theta)^n = cos(n theta) + jsin(n theta)$.



                                  For example, when $n=2$,
                                  $$(cos theta + j sin theta)^2 = cos^2(theta) - sin^2(theta) + j2 sin theta cos theta = cos(2 theta) + jsin(2 theta)$$
                                  implying
                                  $$sin(2theta) = 2sin(theta)cos(theta)$$
                                  $$cos(2theta) = cos^2(theta)-sin^2(theta)$$



                                  You can similarly derive the result for $n=4$.



                                  Anyway, now you can compute
                                  $$tan(4 theta) = {sin(4 theta) over cos(4 theta)} = {4 sin theta cos theta ; (cos^2 theta - sin^2 theta) over (cos^2 theta - sin^2 theta)^2 - 4sin^2 theta cos^2 theta}$$



                                  If you divide the numerator and denominator by $cos^4 theta$, this simplifies to



                                  $$tan(4 theta) = {4 tan theta - 4 tan^3 theta over 1 - 6 tan^2 theta + tan^4 theta}$$



                                  Now, note that when $4 theta = n pi/2$, $tan 4 theta = infty$, i.e. $1 - 6 tan^2 theta + tan^4 theta = 0$. This is exactly the equation you want to solve (with $x = tan theta$), so you get $x = tan (n pi/8)$.






                                  share|cite|improve this answer









                                  $endgroup$
















                                    1












                                    1








                                    1





                                    $begingroup$

                                    $$sin(4 theta) = 2 sin (2 theta) cos(2 theta)$$
                                    $$cos(4 theta) = cos^2(2 theta)-sin^2(2theta)$$
                                    $$sin(2theta) = 2sin(theta)cos(theta)$$
                                    $$cos(2theta) = cos^2(theta)-sin^2(theta)$$



                                    Note that all of these can be derived from de Moivre's theorem $(cos theta + j sin theta)^n = cos(n theta) + jsin(n theta)$.



                                    For example, when $n=2$,
                                    $$(cos theta + j sin theta)^2 = cos^2(theta) - sin^2(theta) + j2 sin theta cos theta = cos(2 theta) + jsin(2 theta)$$
                                    implying
                                    $$sin(2theta) = 2sin(theta)cos(theta)$$
                                    $$cos(2theta) = cos^2(theta)-sin^2(theta)$$



                                    You can similarly derive the result for $n=4$.



                                    Anyway, now you can compute
                                    $$tan(4 theta) = {sin(4 theta) over cos(4 theta)} = {4 sin theta cos theta ; (cos^2 theta - sin^2 theta) over (cos^2 theta - sin^2 theta)^2 - 4sin^2 theta cos^2 theta}$$



                                    If you divide the numerator and denominator by $cos^4 theta$, this simplifies to



                                    $$tan(4 theta) = {4 tan theta - 4 tan^3 theta over 1 - 6 tan^2 theta + tan^4 theta}$$



                                    Now, note that when $4 theta = n pi/2$, $tan 4 theta = infty$, i.e. $1 - 6 tan^2 theta + tan^4 theta = 0$. This is exactly the equation you want to solve (with $x = tan theta$), so you get $x = tan (n pi/8)$.






                                    share|cite|improve this answer









                                    $endgroup$



                                    $$sin(4 theta) = 2 sin (2 theta) cos(2 theta)$$
                                    $$cos(4 theta) = cos^2(2 theta)-sin^2(2theta)$$
                                    $$sin(2theta) = 2sin(theta)cos(theta)$$
                                    $$cos(2theta) = cos^2(theta)-sin^2(theta)$$



                                    Note that all of these can be derived from de Moivre's theorem $(cos theta + j sin theta)^n = cos(n theta) + jsin(n theta)$.



                                    For example, when $n=2$,
                                    $$(cos theta + j sin theta)^2 = cos^2(theta) - sin^2(theta) + j2 sin theta cos theta = cos(2 theta) + jsin(2 theta)$$
                                    implying
                                    $$sin(2theta) = 2sin(theta)cos(theta)$$
                                    $$cos(2theta) = cos^2(theta)-sin^2(theta)$$



                                    You can similarly derive the result for $n=4$.



                                    Anyway, now you can compute
                                    $$tan(4 theta) = {sin(4 theta) over cos(4 theta)} = {4 sin theta cos theta ; (cos^2 theta - sin^2 theta) over (cos^2 theta - sin^2 theta)^2 - 4sin^2 theta cos^2 theta}$$



                                    If you divide the numerator and denominator by $cos^4 theta$, this simplifies to



                                    $$tan(4 theta) = {4 tan theta - 4 tan^3 theta over 1 - 6 tan^2 theta + tan^4 theta}$$



                                    Now, note that when $4 theta = n pi/2$, $tan 4 theta = infty$, i.e. $1 - 6 tan^2 theta + tan^4 theta = 0$. This is exactly the equation you want to solve (with $x = tan theta$), so you get $x = tan (n pi/8)$.







                                    share|cite|improve this answer












                                    share|cite|improve this answer



                                    share|cite|improve this answer










                                    answered Jan 24 at 3:53









                                    Aditya DuaAditya Dua

                                    1,15418




                                    1,15418






























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