Geometric representation of all the points for $cos(z) = 2$, $z in mathbb C$












0












$begingroup$


So I am having trouble visualizing the solutions to
$$cos(z) = 2, z in mathbb{C}$$
I know that the solution of this (like mentioned of here Solving $cos z = 2$ ), but what points does this equation represent on the complex plane? Does it simply represent all the points which are on $x=2$ and $y=0$?










share|cite|improve this question











$endgroup$

















    0












    $begingroup$


    So I am having trouble visualizing the solutions to
    $$cos(z) = 2, z in mathbb{C}$$
    I know that the solution of this (like mentioned of here Solving $cos z = 2$ ), but what points does this equation represent on the complex plane? Does it simply represent all the points which are on $x=2$ and $y=0$?










    share|cite|improve this question











    $endgroup$















      0












      0








      0





      $begingroup$


      So I am having trouble visualizing the solutions to
      $$cos(z) = 2, z in mathbb{C}$$
      I know that the solution of this (like mentioned of here Solving $cos z = 2$ ), but what points does this equation represent on the complex plane? Does it simply represent all the points which are on $x=2$ and $y=0$?










      share|cite|improve this question











      $endgroup$




      So I am having trouble visualizing the solutions to
      $$cos(z) = 2, z in mathbb{C}$$
      I know that the solution of this (like mentioned of here Solving $cos z = 2$ ), but what points does this equation represent on the complex plane? Does it simply represent all the points which are on $x=2$ and $y=0$?







      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 Dec 20 '18 at 16:10









      Andrei

      12.4k21128




      12.4k21128










      asked Dec 20 '18 at 15:52









      daljit97daljit97

      178111




      178111






















          2 Answers
          2






          active

          oldest

          votes


















          1












          $begingroup$

          If $z=x+iy$ then



          $$ cos(x+iy) = cos(x)cos(iy) - sin(x)sin(iy) = cos(x)cosh(y) - isin(x)sinh(y) $$



          Therefore



          begin{align} cos(x)cosh(y) &= 2 \ sin(x)sinh(y) &= 0 end{align}



          Obviously, no real solution exists, so $yne 0 implies sin x = 0$.



          But $cosh (y) > 0, forall y implies cos x > 0 implies cos(x) = 1 implies x = 2npi$, which leaves



          $$ cosh(y) = 2 implies y = pm cosh^{-1}(2) = pm ln (2 + sqrt{3}) $$



          So the solution is



          $$ x = 2npi pm iln(2 + sqrt{3}) $$



          For a visualization, think of the solutions as points on the 2 lines $y = pm ln(2+sqrt{3})$, spaced out evenly by $2npi$






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Why is $implies cos(x) = 1$ true? Where does that follow from? Also why there is no real solution?
            $endgroup$
            – daljit97
            Dec 23 '18 at 17:36








          • 1




            $begingroup$
            If $sin x = 0$ and $cos x > 0$, what does that imply about the value of $cos x$? What's the maximum value of $sin x$ for real $x$?
            $endgroup$
            – Dylan
            Dec 23 '18 at 18:05












          • $begingroup$
            Ok I understand, when $sin x = 0$, $cos x$ can only be 1 or -1. Obvioulsy $|sin x|$ cannot be greater than 1.
            $endgroup$
            – daljit97
            Dec 23 '18 at 18:09












          • $begingroup$
            So the justification behind $sin x =0$ would be that if $sin x sinh y= 0$, either $sin x=0$ or $sinh y =0$. Assuming $sinh y= 0$ implies $y = 2pi n, n in mathbb Z$. Therefore $cosh y = 1$ and $cos x = 2$ which is impossible, therefore $sin x =0$. However, since you said that this is obvious is there a "shortcut" to deduce this?
            $endgroup$
            – daljit97
            Dec 23 '18 at 18:42








          • 1




            $begingroup$
            $sinh y = 0$ only has one solution which is $y=0$
            $endgroup$
            – Dylan
            Dec 23 '18 at 18:45





















          -1












          $begingroup$

          Have you thought about considering the polar form of the equation? It might help you think geometrically. $cos z = text{Re}(e^{iz})=2$. Then it is important that you consider the complex definition of $ln$ (not just the principal value) in the absence of a branch cut. Then it should be clear of the periodicity of your solutions.






          share|cite|improve this answer









          $endgroup$









          • 2




            $begingroup$
            If $z in mathbb C$, then $cos z$ is not the real part of $e^{iz}$, rather $$ e^{iz} = e^{i(x+iy)} = e^{-y}(cos x + isin x) $$ $cos z = frac{e^{iz}+e^{-iz}}{2}$ is not a real-valued function
            $endgroup$
            – Dylan
            Dec 21 '18 at 18:38













          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%2f3047674%2fgeometric-representation-of-all-the-points-for-cosz-2-z-in-mathbb-c%23new-answer', 'question_page');
          }
          );

          Post as a guest















          Required, but never shown

























          2 Answers
          2






          active

          oldest

          votes








          2 Answers
          2






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          1












          $begingroup$

          If $z=x+iy$ then



          $$ cos(x+iy) = cos(x)cos(iy) - sin(x)sin(iy) = cos(x)cosh(y) - isin(x)sinh(y) $$



          Therefore



          begin{align} cos(x)cosh(y) &= 2 \ sin(x)sinh(y) &= 0 end{align}



          Obviously, no real solution exists, so $yne 0 implies sin x = 0$.



          But $cosh (y) > 0, forall y implies cos x > 0 implies cos(x) = 1 implies x = 2npi$, which leaves



          $$ cosh(y) = 2 implies y = pm cosh^{-1}(2) = pm ln (2 + sqrt{3}) $$



          So the solution is



          $$ x = 2npi pm iln(2 + sqrt{3}) $$



          For a visualization, think of the solutions as points on the 2 lines $y = pm ln(2+sqrt{3})$, spaced out evenly by $2npi$






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Why is $implies cos(x) = 1$ true? Where does that follow from? Also why there is no real solution?
            $endgroup$
            – daljit97
            Dec 23 '18 at 17:36








          • 1




            $begingroup$
            If $sin x = 0$ and $cos x > 0$, what does that imply about the value of $cos x$? What's the maximum value of $sin x$ for real $x$?
            $endgroup$
            – Dylan
            Dec 23 '18 at 18:05












          • $begingroup$
            Ok I understand, when $sin x = 0$, $cos x$ can only be 1 or -1. Obvioulsy $|sin x|$ cannot be greater than 1.
            $endgroup$
            – daljit97
            Dec 23 '18 at 18:09












          • $begingroup$
            So the justification behind $sin x =0$ would be that if $sin x sinh y= 0$, either $sin x=0$ or $sinh y =0$. Assuming $sinh y= 0$ implies $y = 2pi n, n in mathbb Z$. Therefore $cosh y = 1$ and $cos x = 2$ which is impossible, therefore $sin x =0$. However, since you said that this is obvious is there a "shortcut" to deduce this?
            $endgroup$
            – daljit97
            Dec 23 '18 at 18:42








          • 1




            $begingroup$
            $sinh y = 0$ only has one solution which is $y=0$
            $endgroup$
            – Dylan
            Dec 23 '18 at 18:45


















          1












          $begingroup$

          If $z=x+iy$ then



          $$ cos(x+iy) = cos(x)cos(iy) - sin(x)sin(iy) = cos(x)cosh(y) - isin(x)sinh(y) $$



          Therefore



          begin{align} cos(x)cosh(y) &= 2 \ sin(x)sinh(y) &= 0 end{align}



          Obviously, no real solution exists, so $yne 0 implies sin x = 0$.



          But $cosh (y) > 0, forall y implies cos x > 0 implies cos(x) = 1 implies x = 2npi$, which leaves



          $$ cosh(y) = 2 implies y = pm cosh^{-1}(2) = pm ln (2 + sqrt{3}) $$



          So the solution is



          $$ x = 2npi pm iln(2 + sqrt{3}) $$



          For a visualization, think of the solutions as points on the 2 lines $y = pm ln(2+sqrt{3})$, spaced out evenly by $2npi$






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Why is $implies cos(x) = 1$ true? Where does that follow from? Also why there is no real solution?
            $endgroup$
            – daljit97
            Dec 23 '18 at 17:36








          • 1




            $begingroup$
            If $sin x = 0$ and $cos x > 0$, what does that imply about the value of $cos x$? What's the maximum value of $sin x$ for real $x$?
            $endgroup$
            – Dylan
            Dec 23 '18 at 18:05












          • $begingroup$
            Ok I understand, when $sin x = 0$, $cos x$ can only be 1 or -1. Obvioulsy $|sin x|$ cannot be greater than 1.
            $endgroup$
            – daljit97
            Dec 23 '18 at 18:09












          • $begingroup$
            So the justification behind $sin x =0$ would be that if $sin x sinh y= 0$, either $sin x=0$ or $sinh y =0$. Assuming $sinh y= 0$ implies $y = 2pi n, n in mathbb Z$. Therefore $cosh y = 1$ and $cos x = 2$ which is impossible, therefore $sin x =0$. However, since you said that this is obvious is there a "shortcut" to deduce this?
            $endgroup$
            – daljit97
            Dec 23 '18 at 18:42








          • 1




            $begingroup$
            $sinh y = 0$ only has one solution which is $y=0$
            $endgroup$
            – Dylan
            Dec 23 '18 at 18:45
















          1












          1








          1





          $begingroup$

          If $z=x+iy$ then



          $$ cos(x+iy) = cos(x)cos(iy) - sin(x)sin(iy) = cos(x)cosh(y) - isin(x)sinh(y) $$



          Therefore



          begin{align} cos(x)cosh(y) &= 2 \ sin(x)sinh(y) &= 0 end{align}



          Obviously, no real solution exists, so $yne 0 implies sin x = 0$.



          But $cosh (y) > 0, forall y implies cos x > 0 implies cos(x) = 1 implies x = 2npi$, which leaves



          $$ cosh(y) = 2 implies y = pm cosh^{-1}(2) = pm ln (2 + sqrt{3}) $$



          So the solution is



          $$ x = 2npi pm iln(2 + sqrt{3}) $$



          For a visualization, think of the solutions as points on the 2 lines $y = pm ln(2+sqrt{3})$, spaced out evenly by $2npi$






          share|cite|improve this answer











          $endgroup$



          If $z=x+iy$ then



          $$ cos(x+iy) = cos(x)cos(iy) - sin(x)sin(iy) = cos(x)cosh(y) - isin(x)sinh(y) $$



          Therefore



          begin{align} cos(x)cosh(y) &= 2 \ sin(x)sinh(y) &= 0 end{align}



          Obviously, no real solution exists, so $yne 0 implies sin x = 0$.



          But $cosh (y) > 0, forall y implies cos x > 0 implies cos(x) = 1 implies x = 2npi$, which leaves



          $$ cosh(y) = 2 implies y = pm cosh^{-1}(2) = pm ln (2 + sqrt{3}) $$



          So the solution is



          $$ x = 2npi pm iln(2 + sqrt{3}) $$



          For a visualization, think of the solutions as points on the 2 lines $y = pm ln(2+sqrt{3})$, spaced out evenly by $2npi$







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Dec 23 '18 at 18:14

























          answered Dec 21 '18 at 18:36









          DylanDylan

          13.3k31027




          13.3k31027












          • $begingroup$
            Why is $implies cos(x) = 1$ true? Where does that follow from? Also why there is no real solution?
            $endgroup$
            – daljit97
            Dec 23 '18 at 17:36








          • 1




            $begingroup$
            If $sin x = 0$ and $cos x > 0$, what does that imply about the value of $cos x$? What's the maximum value of $sin x$ for real $x$?
            $endgroup$
            – Dylan
            Dec 23 '18 at 18:05












          • $begingroup$
            Ok I understand, when $sin x = 0$, $cos x$ can only be 1 or -1. Obvioulsy $|sin x|$ cannot be greater than 1.
            $endgroup$
            – daljit97
            Dec 23 '18 at 18:09












          • $begingroup$
            So the justification behind $sin x =0$ would be that if $sin x sinh y= 0$, either $sin x=0$ or $sinh y =0$. Assuming $sinh y= 0$ implies $y = 2pi n, n in mathbb Z$. Therefore $cosh y = 1$ and $cos x = 2$ which is impossible, therefore $sin x =0$. However, since you said that this is obvious is there a "shortcut" to deduce this?
            $endgroup$
            – daljit97
            Dec 23 '18 at 18:42








          • 1




            $begingroup$
            $sinh y = 0$ only has one solution which is $y=0$
            $endgroup$
            – Dylan
            Dec 23 '18 at 18:45




















          • $begingroup$
            Why is $implies cos(x) = 1$ true? Where does that follow from? Also why there is no real solution?
            $endgroup$
            – daljit97
            Dec 23 '18 at 17:36








          • 1




            $begingroup$
            If $sin x = 0$ and $cos x > 0$, what does that imply about the value of $cos x$? What's the maximum value of $sin x$ for real $x$?
            $endgroup$
            – Dylan
            Dec 23 '18 at 18:05












          • $begingroup$
            Ok I understand, when $sin x = 0$, $cos x$ can only be 1 or -1. Obvioulsy $|sin x|$ cannot be greater than 1.
            $endgroup$
            – daljit97
            Dec 23 '18 at 18:09












          • $begingroup$
            So the justification behind $sin x =0$ would be that if $sin x sinh y= 0$, either $sin x=0$ or $sinh y =0$. Assuming $sinh y= 0$ implies $y = 2pi n, n in mathbb Z$. Therefore $cosh y = 1$ and $cos x = 2$ which is impossible, therefore $sin x =0$. However, since you said that this is obvious is there a "shortcut" to deduce this?
            $endgroup$
            – daljit97
            Dec 23 '18 at 18:42








          • 1




            $begingroup$
            $sinh y = 0$ only has one solution which is $y=0$
            $endgroup$
            – Dylan
            Dec 23 '18 at 18:45


















          $begingroup$
          Why is $implies cos(x) = 1$ true? Where does that follow from? Also why there is no real solution?
          $endgroup$
          – daljit97
          Dec 23 '18 at 17:36






          $begingroup$
          Why is $implies cos(x) = 1$ true? Where does that follow from? Also why there is no real solution?
          $endgroup$
          – daljit97
          Dec 23 '18 at 17:36






          1




          1




          $begingroup$
          If $sin x = 0$ and $cos x > 0$, what does that imply about the value of $cos x$? What's the maximum value of $sin x$ for real $x$?
          $endgroup$
          – Dylan
          Dec 23 '18 at 18:05






          $begingroup$
          If $sin x = 0$ and $cos x > 0$, what does that imply about the value of $cos x$? What's the maximum value of $sin x$ for real $x$?
          $endgroup$
          – Dylan
          Dec 23 '18 at 18:05














          $begingroup$
          Ok I understand, when $sin x = 0$, $cos x$ can only be 1 or -1. Obvioulsy $|sin x|$ cannot be greater than 1.
          $endgroup$
          – daljit97
          Dec 23 '18 at 18:09






          $begingroup$
          Ok I understand, when $sin x = 0$, $cos x$ can only be 1 or -1. Obvioulsy $|sin x|$ cannot be greater than 1.
          $endgroup$
          – daljit97
          Dec 23 '18 at 18:09














          $begingroup$
          So the justification behind $sin x =0$ would be that if $sin x sinh y= 0$, either $sin x=0$ or $sinh y =0$. Assuming $sinh y= 0$ implies $y = 2pi n, n in mathbb Z$. Therefore $cosh y = 1$ and $cos x = 2$ which is impossible, therefore $sin x =0$. However, since you said that this is obvious is there a "shortcut" to deduce this?
          $endgroup$
          – daljit97
          Dec 23 '18 at 18:42






          $begingroup$
          So the justification behind $sin x =0$ would be that if $sin x sinh y= 0$, either $sin x=0$ or $sinh y =0$. Assuming $sinh y= 0$ implies $y = 2pi n, n in mathbb Z$. Therefore $cosh y = 1$ and $cos x = 2$ which is impossible, therefore $sin x =0$. However, since you said that this is obvious is there a "shortcut" to deduce this?
          $endgroup$
          – daljit97
          Dec 23 '18 at 18:42






          1




          1




          $begingroup$
          $sinh y = 0$ only has one solution which is $y=0$
          $endgroup$
          – Dylan
          Dec 23 '18 at 18:45






          $begingroup$
          $sinh y = 0$ only has one solution which is $y=0$
          $endgroup$
          – Dylan
          Dec 23 '18 at 18:45













          -1












          $begingroup$

          Have you thought about considering the polar form of the equation? It might help you think geometrically. $cos z = text{Re}(e^{iz})=2$. Then it is important that you consider the complex definition of $ln$ (not just the principal value) in the absence of a branch cut. Then it should be clear of the periodicity of your solutions.






          share|cite|improve this answer









          $endgroup$









          • 2




            $begingroup$
            If $z in mathbb C$, then $cos z$ is not the real part of $e^{iz}$, rather $$ e^{iz} = e^{i(x+iy)} = e^{-y}(cos x + isin x) $$ $cos z = frac{e^{iz}+e^{-iz}}{2}$ is not a real-valued function
            $endgroup$
            – Dylan
            Dec 21 '18 at 18:38


















          -1












          $begingroup$

          Have you thought about considering the polar form of the equation? It might help you think geometrically. $cos z = text{Re}(e^{iz})=2$. Then it is important that you consider the complex definition of $ln$ (not just the principal value) in the absence of a branch cut. Then it should be clear of the periodicity of your solutions.






          share|cite|improve this answer









          $endgroup$









          • 2




            $begingroup$
            If $z in mathbb C$, then $cos z$ is not the real part of $e^{iz}$, rather $$ e^{iz} = e^{i(x+iy)} = e^{-y}(cos x + isin x) $$ $cos z = frac{e^{iz}+e^{-iz}}{2}$ is not a real-valued function
            $endgroup$
            – Dylan
            Dec 21 '18 at 18:38
















          -1












          -1








          -1





          $begingroup$

          Have you thought about considering the polar form of the equation? It might help you think geometrically. $cos z = text{Re}(e^{iz})=2$. Then it is important that you consider the complex definition of $ln$ (not just the principal value) in the absence of a branch cut. Then it should be clear of the periodicity of your solutions.






          share|cite|improve this answer









          $endgroup$



          Have you thought about considering the polar form of the equation? It might help you think geometrically. $cos z = text{Re}(e^{iz})=2$. Then it is important that you consider the complex definition of $ln$ (not just the principal value) in the absence of a branch cut. Then it should be clear of the periodicity of your solutions.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Dec 20 '18 at 16:56









          Joel BiffinJoel Biffin

          966




          966








          • 2




            $begingroup$
            If $z in mathbb C$, then $cos z$ is not the real part of $e^{iz}$, rather $$ e^{iz} = e^{i(x+iy)} = e^{-y}(cos x + isin x) $$ $cos z = frac{e^{iz}+e^{-iz}}{2}$ is not a real-valued function
            $endgroup$
            – Dylan
            Dec 21 '18 at 18:38
















          • 2




            $begingroup$
            If $z in mathbb C$, then $cos z$ is not the real part of $e^{iz}$, rather $$ e^{iz} = e^{i(x+iy)} = e^{-y}(cos x + isin x) $$ $cos z = frac{e^{iz}+e^{-iz}}{2}$ is not a real-valued function
            $endgroup$
            – Dylan
            Dec 21 '18 at 18:38










          2




          2




          $begingroup$
          If $z in mathbb C$, then $cos z$ is not the real part of $e^{iz}$, rather $$ e^{iz} = e^{i(x+iy)} = e^{-y}(cos x + isin x) $$ $cos z = frac{e^{iz}+e^{-iz}}{2}$ is not a real-valued function
          $endgroup$
          – Dylan
          Dec 21 '18 at 18:38






          $begingroup$
          If $z in mathbb C$, then $cos z$ is not the real part of $e^{iz}$, rather $$ e^{iz} = e^{i(x+iy)} = e^{-y}(cos x + isin x) $$ $cos z = frac{e^{iz}+e^{-iz}}{2}$ is not a real-valued function
          $endgroup$
          – Dylan
          Dec 21 '18 at 18:38




















          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%2f3047674%2fgeometric-representation-of-all-the-points-for-cosz-2-z-in-mathbb-c%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

          Wiesbaden

          Marschland

          Dieringhausen