$|z|=z^5$ How many solutions does this equation have?












1












$begingroup$


Multiple choice question : How many solutions does the equation $|z|=z^5$ have?



A - $1$ solution



B - $2$ solutions



C - $5$ solutions



D - $6$ solutions



These were the $4$ possible answers.



I started by allowing $z^5in Bbb R$.



$|z|=|z|^5$ which led me to $|z|=1$ or $|z|=0$.



I found $2$ obvious solutions, $z=1$ and $z=0$
However I couldn't find any other complex solutions; and the B answer was not correct.



I have also tried to expand with $z=a+ib$ but couldn't find anything, still.



The correct answer was D but I couldn't understand why.



Excuse my English, I am not used to doing maths in English.










share|cite|improve this question











$endgroup$

















    1












    $begingroup$


    Multiple choice question : How many solutions does the equation $|z|=z^5$ have?



    A - $1$ solution



    B - $2$ solutions



    C - $5$ solutions



    D - $6$ solutions



    These were the $4$ possible answers.



    I started by allowing $z^5in Bbb R$.



    $|z|=|z|^5$ which led me to $|z|=1$ or $|z|=0$.



    I found $2$ obvious solutions, $z=1$ and $z=0$
    However I couldn't find any other complex solutions; and the B answer was not correct.



    I have also tried to expand with $z=a+ib$ but couldn't find anything, still.



    The correct answer was D but I couldn't understand why.



    Excuse my English, I am not used to doing maths in English.










    share|cite|improve this question











    $endgroup$















      1












      1








      1





      $begingroup$


      Multiple choice question : How many solutions does the equation $|z|=z^5$ have?



      A - $1$ solution



      B - $2$ solutions



      C - $5$ solutions



      D - $6$ solutions



      These were the $4$ possible answers.



      I started by allowing $z^5in Bbb R$.



      $|z|=|z|^5$ which led me to $|z|=1$ or $|z|=0$.



      I found $2$ obvious solutions, $z=1$ and $z=0$
      However I couldn't find any other complex solutions; and the B answer was not correct.



      I have also tried to expand with $z=a+ib$ but couldn't find anything, still.



      The correct answer was D but I couldn't understand why.



      Excuse my English, I am not used to doing maths in English.










      share|cite|improve this question











      $endgroup$




      Multiple choice question : How many solutions does the equation $|z|=z^5$ have?



      A - $1$ solution



      B - $2$ solutions



      C - $5$ solutions



      D - $6$ solutions



      These were the $4$ possible answers.



      I started by allowing $z^5in Bbb R$.



      $|z|=|z|^5$ which led me to $|z|=1$ or $|z|=0$.



      I found $2$ obvious solutions, $z=1$ and $z=0$
      However I couldn't find any other complex solutions; and the B answer was not correct.



      I have also tried to expand with $z=a+ib$ but couldn't find anything, still.



      The correct answer was D but I couldn't understand why.



      Excuse my English, I am not used to doing maths in English.







      complex-analysis complex-numbers






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Jan 6 at 17:39









      José Carlos Santos

      173k23133242




      173k23133242










      asked Jan 6 at 17:21









      CaioCaio

      61




      61






















          3 Answers
          3






          active

          oldest

          votes


















          4












          $begingroup$

          Any $5$-th root of $1$ is also a solution. With zero, this makes $6$ solutions in total.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            You mean 4th root, no ? btw, $z^5$ must be real, so not that much solution...
            $endgroup$
            – NewMath
            Jan 6 at 17:29










          • $begingroup$
            @NewMath For any $5$-th root $z$ of $1$ you have $z^5=1$ and $vert z vert =1$. Hence this is a root of the equation provided.
            $endgroup$
            – mathcounterexamples.net
            Jan 6 at 17:31



















          2












          $begingroup$

          It is indeed true that$$lvert zrvert=z^5implieslvert zrvert=0veelvert zrvert=1.$$However, I don't understand your proof. You can prove it as follows:begin{align}lvert zrvert=z^5implies&bigllvertlvert zrvertbigllvert=lvert z^5rvert\iff&lvert zrvert=lvert zrvert^5\iff&lvert zrvert=0veelvert zrvert^4=1\iff&lvert zrvert=0veelvert zrvert=1.end{align}If $lvert zrvert=0$, then $z=0$ and $0$ is indeed a solution. And if $lvert zrvert=1$, the equation becomes $z^5=1$. So, take the $5$ fifth roots of $1$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Shouldn't it be $|z|=|z|^5iff |z|=0vee |z|^4=1$ instead of $|z|=1$ ?
            $endgroup$
            – NewMath
            Jan 6 at 17:31












          • $begingroup$
            I've edited my answer. I hope that everything is clear now.
            $endgroup$
            – José Carlos Santos
            Jan 6 at 17:35










          • $begingroup$
            @NewMath While indeed $|z|=|z|^5iff |z|=0vee |z|^4=1$, you want to solve the stronger $|z|=z^5$.
            $endgroup$
            – Hagen von Eitzen
            Jan 6 at 17:48



















          0












          $begingroup$

          $$
          eqalign{
          & left| z right| = z^{,5} ,quad mathop Rightarrow limits^{z = Ae^{,i,alpha } } quad A = A^{,5} e^{,i,5alpha } quad Rightarrow cr
          & Rightarrow quad left{ {matrix{
          {A = A^{,5} } cr
          {e^{,i,5alpha } = 1} cr
          } } right.quad Rightarrow quad left{ {matrix{
          {A = 0,1} cr
          {alpha = 2kpi /5} cr
          } } right.quad Rightarrow cr
          & Rightarrow quad z = 0,e^{,2kpi /5} ;left| {;k = 0, cdots ,4} right.quad Rightarrow quad 6quad sol. cr}
          $$






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            thank you all this was very helpful
            $endgroup$
            – Caio
            Jan 6 at 18:01










          • $begingroup$
            @Caio: wish it is also clear which is the step-wise approach you should take not to entangle in the common "traps" of complex calculus
            $endgroup$
            – G Cab
            Jan 6 at 18:22












          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%2f3064138%2fz-z5-how-many-solutions-does-this-equation-have%23new-answer', 'question_page');
          }
          );

          Post as a guest















          Required, but never shown

























          3 Answers
          3






          active

          oldest

          votes








          3 Answers
          3






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          4












          $begingroup$

          Any $5$-th root of $1$ is also a solution. With zero, this makes $6$ solutions in total.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            You mean 4th root, no ? btw, $z^5$ must be real, so not that much solution...
            $endgroup$
            – NewMath
            Jan 6 at 17:29










          • $begingroup$
            @NewMath For any $5$-th root $z$ of $1$ you have $z^5=1$ and $vert z vert =1$. Hence this is a root of the equation provided.
            $endgroup$
            – mathcounterexamples.net
            Jan 6 at 17:31
















          4












          $begingroup$

          Any $5$-th root of $1$ is also a solution. With zero, this makes $6$ solutions in total.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            You mean 4th root, no ? btw, $z^5$ must be real, so not that much solution...
            $endgroup$
            – NewMath
            Jan 6 at 17:29










          • $begingroup$
            @NewMath For any $5$-th root $z$ of $1$ you have $z^5=1$ and $vert z vert =1$. Hence this is a root of the equation provided.
            $endgroup$
            – mathcounterexamples.net
            Jan 6 at 17:31














          4












          4








          4





          $begingroup$

          Any $5$-th root of $1$ is also a solution. With zero, this makes $6$ solutions in total.






          share|cite|improve this answer









          $endgroup$



          Any $5$-th root of $1$ is also a solution. With zero, this makes $6$ solutions in total.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Jan 6 at 17:25









          mathcounterexamples.netmathcounterexamples.net

          26.9k22158




          26.9k22158












          • $begingroup$
            You mean 4th root, no ? btw, $z^5$ must be real, so not that much solution...
            $endgroup$
            – NewMath
            Jan 6 at 17:29










          • $begingroup$
            @NewMath For any $5$-th root $z$ of $1$ you have $z^5=1$ and $vert z vert =1$. Hence this is a root of the equation provided.
            $endgroup$
            – mathcounterexamples.net
            Jan 6 at 17:31


















          • $begingroup$
            You mean 4th root, no ? btw, $z^5$ must be real, so not that much solution...
            $endgroup$
            – NewMath
            Jan 6 at 17:29










          • $begingroup$
            @NewMath For any $5$-th root $z$ of $1$ you have $z^5=1$ and $vert z vert =1$. Hence this is a root of the equation provided.
            $endgroup$
            – mathcounterexamples.net
            Jan 6 at 17:31
















          $begingroup$
          You mean 4th root, no ? btw, $z^5$ must be real, so not that much solution...
          $endgroup$
          – NewMath
          Jan 6 at 17:29




          $begingroup$
          You mean 4th root, no ? btw, $z^5$ must be real, so not that much solution...
          $endgroup$
          – NewMath
          Jan 6 at 17:29












          $begingroup$
          @NewMath For any $5$-th root $z$ of $1$ you have $z^5=1$ and $vert z vert =1$. Hence this is a root of the equation provided.
          $endgroup$
          – mathcounterexamples.net
          Jan 6 at 17:31




          $begingroup$
          @NewMath For any $5$-th root $z$ of $1$ you have $z^5=1$ and $vert z vert =1$. Hence this is a root of the equation provided.
          $endgroup$
          – mathcounterexamples.net
          Jan 6 at 17:31











          2












          $begingroup$

          It is indeed true that$$lvert zrvert=z^5implieslvert zrvert=0veelvert zrvert=1.$$However, I don't understand your proof. You can prove it as follows:begin{align}lvert zrvert=z^5implies&bigllvertlvert zrvertbigllvert=lvert z^5rvert\iff&lvert zrvert=lvert zrvert^5\iff&lvert zrvert=0veelvert zrvert^4=1\iff&lvert zrvert=0veelvert zrvert=1.end{align}If $lvert zrvert=0$, then $z=0$ and $0$ is indeed a solution. And if $lvert zrvert=1$, the equation becomes $z^5=1$. So, take the $5$ fifth roots of $1$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Shouldn't it be $|z|=|z|^5iff |z|=0vee |z|^4=1$ instead of $|z|=1$ ?
            $endgroup$
            – NewMath
            Jan 6 at 17:31












          • $begingroup$
            I've edited my answer. I hope that everything is clear now.
            $endgroup$
            – José Carlos Santos
            Jan 6 at 17:35










          • $begingroup$
            @NewMath While indeed $|z|=|z|^5iff |z|=0vee |z|^4=1$, you want to solve the stronger $|z|=z^5$.
            $endgroup$
            – Hagen von Eitzen
            Jan 6 at 17:48
















          2












          $begingroup$

          It is indeed true that$$lvert zrvert=z^5implieslvert zrvert=0veelvert zrvert=1.$$However, I don't understand your proof. You can prove it as follows:begin{align}lvert zrvert=z^5implies&bigllvertlvert zrvertbigllvert=lvert z^5rvert\iff&lvert zrvert=lvert zrvert^5\iff&lvert zrvert=0veelvert zrvert^4=1\iff&lvert zrvert=0veelvert zrvert=1.end{align}If $lvert zrvert=0$, then $z=0$ and $0$ is indeed a solution. And if $lvert zrvert=1$, the equation becomes $z^5=1$. So, take the $5$ fifth roots of $1$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Shouldn't it be $|z|=|z|^5iff |z|=0vee |z|^4=1$ instead of $|z|=1$ ?
            $endgroup$
            – NewMath
            Jan 6 at 17:31












          • $begingroup$
            I've edited my answer. I hope that everything is clear now.
            $endgroup$
            – José Carlos Santos
            Jan 6 at 17:35










          • $begingroup$
            @NewMath While indeed $|z|=|z|^5iff |z|=0vee |z|^4=1$, you want to solve the stronger $|z|=z^5$.
            $endgroup$
            – Hagen von Eitzen
            Jan 6 at 17:48














          2












          2








          2





          $begingroup$

          It is indeed true that$$lvert zrvert=z^5implieslvert zrvert=0veelvert zrvert=1.$$However, I don't understand your proof. You can prove it as follows:begin{align}lvert zrvert=z^5implies&bigllvertlvert zrvertbigllvert=lvert z^5rvert\iff&lvert zrvert=lvert zrvert^5\iff&lvert zrvert=0veelvert zrvert^4=1\iff&lvert zrvert=0veelvert zrvert=1.end{align}If $lvert zrvert=0$, then $z=0$ and $0$ is indeed a solution. And if $lvert zrvert=1$, the equation becomes $z^5=1$. So, take the $5$ fifth roots of $1$.






          share|cite|improve this answer











          $endgroup$



          It is indeed true that$$lvert zrvert=z^5implieslvert zrvert=0veelvert zrvert=1.$$However, I don't understand your proof. You can prove it as follows:begin{align}lvert zrvert=z^5implies&bigllvertlvert zrvertbigllvert=lvert z^5rvert\iff&lvert zrvert=lvert zrvert^5\iff&lvert zrvert=0veelvert zrvert^4=1\iff&lvert zrvert=0veelvert zrvert=1.end{align}If $lvert zrvert=0$, then $z=0$ and $0$ is indeed a solution. And if $lvert zrvert=1$, the equation becomes $z^5=1$. So, take the $5$ fifth roots of $1$.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Jan 6 at 17:34

























          answered Jan 6 at 17:28









          José Carlos SantosJosé Carlos Santos

          173k23133242




          173k23133242












          • $begingroup$
            Shouldn't it be $|z|=|z|^5iff |z|=0vee |z|^4=1$ instead of $|z|=1$ ?
            $endgroup$
            – NewMath
            Jan 6 at 17:31












          • $begingroup$
            I've edited my answer. I hope that everything is clear now.
            $endgroup$
            – José Carlos Santos
            Jan 6 at 17:35










          • $begingroup$
            @NewMath While indeed $|z|=|z|^5iff |z|=0vee |z|^4=1$, you want to solve the stronger $|z|=z^5$.
            $endgroup$
            – Hagen von Eitzen
            Jan 6 at 17:48


















          • $begingroup$
            Shouldn't it be $|z|=|z|^5iff |z|=0vee |z|^4=1$ instead of $|z|=1$ ?
            $endgroup$
            – NewMath
            Jan 6 at 17:31












          • $begingroup$
            I've edited my answer. I hope that everything is clear now.
            $endgroup$
            – José Carlos Santos
            Jan 6 at 17:35










          • $begingroup$
            @NewMath While indeed $|z|=|z|^5iff |z|=0vee |z|^4=1$, you want to solve the stronger $|z|=z^5$.
            $endgroup$
            – Hagen von Eitzen
            Jan 6 at 17:48
















          $begingroup$
          Shouldn't it be $|z|=|z|^5iff |z|=0vee |z|^4=1$ instead of $|z|=1$ ?
          $endgroup$
          – NewMath
          Jan 6 at 17:31






          $begingroup$
          Shouldn't it be $|z|=|z|^5iff |z|=0vee |z|^4=1$ instead of $|z|=1$ ?
          $endgroup$
          – NewMath
          Jan 6 at 17:31














          $begingroup$
          I've edited my answer. I hope that everything is clear now.
          $endgroup$
          – José Carlos Santos
          Jan 6 at 17:35




          $begingroup$
          I've edited my answer. I hope that everything is clear now.
          $endgroup$
          – José Carlos Santos
          Jan 6 at 17:35












          $begingroup$
          @NewMath While indeed $|z|=|z|^5iff |z|=0vee |z|^4=1$, you want to solve the stronger $|z|=z^5$.
          $endgroup$
          – Hagen von Eitzen
          Jan 6 at 17:48




          $begingroup$
          @NewMath While indeed $|z|=|z|^5iff |z|=0vee |z|^4=1$, you want to solve the stronger $|z|=z^5$.
          $endgroup$
          – Hagen von Eitzen
          Jan 6 at 17:48











          0












          $begingroup$

          $$
          eqalign{
          & left| z right| = z^{,5} ,quad mathop Rightarrow limits^{z = Ae^{,i,alpha } } quad A = A^{,5} e^{,i,5alpha } quad Rightarrow cr
          & Rightarrow quad left{ {matrix{
          {A = A^{,5} } cr
          {e^{,i,5alpha } = 1} cr
          } } right.quad Rightarrow quad left{ {matrix{
          {A = 0,1} cr
          {alpha = 2kpi /5} cr
          } } right.quad Rightarrow cr
          & Rightarrow quad z = 0,e^{,2kpi /5} ;left| {;k = 0, cdots ,4} right.quad Rightarrow quad 6quad sol. cr}
          $$






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            thank you all this was very helpful
            $endgroup$
            – Caio
            Jan 6 at 18:01










          • $begingroup$
            @Caio: wish it is also clear which is the step-wise approach you should take not to entangle in the common "traps" of complex calculus
            $endgroup$
            – G Cab
            Jan 6 at 18:22
















          0












          $begingroup$

          $$
          eqalign{
          & left| z right| = z^{,5} ,quad mathop Rightarrow limits^{z = Ae^{,i,alpha } } quad A = A^{,5} e^{,i,5alpha } quad Rightarrow cr
          & Rightarrow quad left{ {matrix{
          {A = A^{,5} } cr
          {e^{,i,5alpha } = 1} cr
          } } right.quad Rightarrow quad left{ {matrix{
          {A = 0,1} cr
          {alpha = 2kpi /5} cr
          } } right.quad Rightarrow cr
          & Rightarrow quad z = 0,e^{,2kpi /5} ;left| {;k = 0, cdots ,4} right.quad Rightarrow quad 6quad sol. cr}
          $$






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            thank you all this was very helpful
            $endgroup$
            – Caio
            Jan 6 at 18:01










          • $begingroup$
            @Caio: wish it is also clear which is the step-wise approach you should take not to entangle in the common "traps" of complex calculus
            $endgroup$
            – G Cab
            Jan 6 at 18:22














          0












          0








          0





          $begingroup$

          $$
          eqalign{
          & left| z right| = z^{,5} ,quad mathop Rightarrow limits^{z = Ae^{,i,alpha } } quad A = A^{,5} e^{,i,5alpha } quad Rightarrow cr
          & Rightarrow quad left{ {matrix{
          {A = A^{,5} } cr
          {e^{,i,5alpha } = 1} cr
          } } right.quad Rightarrow quad left{ {matrix{
          {A = 0,1} cr
          {alpha = 2kpi /5} cr
          } } right.quad Rightarrow cr
          & Rightarrow quad z = 0,e^{,2kpi /5} ;left| {;k = 0, cdots ,4} right.quad Rightarrow quad 6quad sol. cr}
          $$






          share|cite|improve this answer









          $endgroup$



          $$
          eqalign{
          & left| z right| = z^{,5} ,quad mathop Rightarrow limits^{z = Ae^{,i,alpha } } quad A = A^{,5} e^{,i,5alpha } quad Rightarrow cr
          & Rightarrow quad left{ {matrix{
          {A = A^{,5} } cr
          {e^{,i,5alpha } = 1} cr
          } } right.quad Rightarrow quad left{ {matrix{
          {A = 0,1} cr
          {alpha = 2kpi /5} cr
          } } right.quad Rightarrow cr
          & Rightarrow quad z = 0,e^{,2kpi /5} ;left| {;k = 0, cdots ,4} right.quad Rightarrow quad 6quad sol. cr}
          $$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Jan 6 at 17:36









          G CabG Cab

          20.4k31341




          20.4k31341












          • $begingroup$
            thank you all this was very helpful
            $endgroup$
            – Caio
            Jan 6 at 18:01










          • $begingroup$
            @Caio: wish it is also clear which is the step-wise approach you should take not to entangle in the common "traps" of complex calculus
            $endgroup$
            – G Cab
            Jan 6 at 18:22


















          • $begingroup$
            thank you all this was very helpful
            $endgroup$
            – Caio
            Jan 6 at 18:01










          • $begingroup$
            @Caio: wish it is also clear which is the step-wise approach you should take not to entangle in the common "traps" of complex calculus
            $endgroup$
            – G Cab
            Jan 6 at 18:22
















          $begingroup$
          thank you all this was very helpful
          $endgroup$
          – Caio
          Jan 6 at 18:01




          $begingroup$
          thank you all this was very helpful
          $endgroup$
          – Caio
          Jan 6 at 18:01












          $begingroup$
          @Caio: wish it is also clear which is the step-wise approach you should take not to entangle in the common "traps" of complex calculus
          $endgroup$
          – G Cab
          Jan 6 at 18:22




          $begingroup$
          @Caio: wish it is also clear which is the step-wise approach you should take not to entangle in the common "traps" of complex calculus
          $endgroup$
          – G Cab
          Jan 6 at 18:22


















          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%2f3064138%2fz-z5-how-many-solutions-does-this-equation-have%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

          How do I know what Microsoft account the skydrive app is syncing to?

          When does type information flow backwards in C++?

          Grease: Live!