What is the value of $cosh(sqrt{i})$?












0












$begingroup$



I am puzzling about the value of



$$cosh(sqrt{i})$$




I know that



$$sqrt{i} = sqrt{frac{1}{2}}+isqrt{frac{1}{2}}$$



But how to go on with that? Are there also multiple values?



Thank you all in advance!










share|cite|improve this question











$endgroup$












  • $begingroup$
    We need some more context. $cosh(sqrt{i})$ is an expression: are you trying to find its value? If so, you don't use the language of "solution", but of "evaluating". On the other hand, if there's an equals sign, and you're trying to find the value of a variable, you "solve it", you do not "evaluate it".
    $endgroup$
    – Adrian Keister
    Jan 30 at 16:09










  • $begingroup$
    Do you know the formula for $cosh?$ If so, what difficulty are you having? If by $sqrt{i}$ you mean the principal value, then there is only one value of the expression.
    $endgroup$
    – saulspatz
    Jan 30 at 16:12
















0












$begingroup$



I am puzzling about the value of



$$cosh(sqrt{i})$$




I know that



$$sqrt{i} = sqrt{frac{1}{2}}+isqrt{frac{1}{2}}$$



But how to go on with that? Are there also multiple values?



Thank you all in advance!










share|cite|improve this question











$endgroup$












  • $begingroup$
    We need some more context. $cosh(sqrt{i})$ is an expression: are you trying to find its value? If so, you don't use the language of "solution", but of "evaluating". On the other hand, if there's an equals sign, and you're trying to find the value of a variable, you "solve it", you do not "evaluate it".
    $endgroup$
    – Adrian Keister
    Jan 30 at 16:09










  • $begingroup$
    Do you know the formula for $cosh?$ If so, what difficulty are you having? If by $sqrt{i}$ you mean the principal value, then there is only one value of the expression.
    $endgroup$
    – saulspatz
    Jan 30 at 16:12














0












0








0





$begingroup$



I am puzzling about the value of



$$cosh(sqrt{i})$$




I know that



$$sqrt{i} = sqrt{frac{1}{2}}+isqrt{frac{1}{2}}$$



But how to go on with that? Are there also multiple values?



Thank you all in advance!










share|cite|improve this question











$endgroup$





I am puzzling about the value of



$$cosh(sqrt{i})$$




I know that



$$sqrt{i} = sqrt{frac{1}{2}}+isqrt{frac{1}{2}}$$



But how to go on with that? Are there also multiple values?



Thank you all in advance!







hyperbolic-functions






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 30 at 17:25









Blue

49.4k870157




49.4k870157










asked Jan 30 at 16:07









Max MustermannMax Mustermann

1




1












  • $begingroup$
    We need some more context. $cosh(sqrt{i})$ is an expression: are you trying to find its value? If so, you don't use the language of "solution", but of "evaluating". On the other hand, if there's an equals sign, and you're trying to find the value of a variable, you "solve it", you do not "evaluate it".
    $endgroup$
    – Adrian Keister
    Jan 30 at 16:09










  • $begingroup$
    Do you know the formula for $cosh?$ If so, what difficulty are you having? If by $sqrt{i}$ you mean the principal value, then there is only one value of the expression.
    $endgroup$
    – saulspatz
    Jan 30 at 16:12


















  • $begingroup$
    We need some more context. $cosh(sqrt{i})$ is an expression: are you trying to find its value? If so, you don't use the language of "solution", but of "evaluating". On the other hand, if there's an equals sign, and you're trying to find the value of a variable, you "solve it", you do not "evaluate it".
    $endgroup$
    – Adrian Keister
    Jan 30 at 16:09










  • $begingroup$
    Do you know the formula for $cosh?$ If so, what difficulty are you having? If by $sqrt{i}$ you mean the principal value, then there is only one value of the expression.
    $endgroup$
    – saulspatz
    Jan 30 at 16:12
















$begingroup$
We need some more context. $cosh(sqrt{i})$ is an expression: are you trying to find its value? If so, you don't use the language of "solution", but of "evaluating". On the other hand, if there's an equals sign, and you're trying to find the value of a variable, you "solve it", you do not "evaluate it".
$endgroup$
– Adrian Keister
Jan 30 at 16:09




$begingroup$
We need some more context. $cosh(sqrt{i})$ is an expression: are you trying to find its value? If so, you don't use the language of "solution", but of "evaluating". On the other hand, if there's an equals sign, and you're trying to find the value of a variable, you "solve it", you do not "evaluate it".
$endgroup$
– Adrian Keister
Jan 30 at 16:09












$begingroup$
Do you know the formula for $cosh?$ If so, what difficulty are you having? If by $sqrt{i}$ you mean the principal value, then there is only one value of the expression.
$endgroup$
– saulspatz
Jan 30 at 16:12




$begingroup$
Do you know the formula for $cosh?$ If so, what difficulty are you having? If by $sqrt{i}$ you mean the principal value, then there is only one value of the expression.
$endgroup$
– saulspatz
Jan 30 at 16:12










3 Answers
3






active

oldest

votes


















1












$begingroup$

Note that there are two possible values for $sqrt{i}$, namely, $pmleft(frac{1}{sqrt{2}}+frac{i}{sqrt{2}}right)$. But since $cosh$ is an even function, it doesn't matter which we choose. So we will work with finding $coshleft(frac{1}{sqrt{2}}+frac{i}{sqrt{2}}right)$.



One easily verifies that $cosh(a+bi)=cosh a cos b + isinh asin b$. Thus



$$coshleft(tfrac{1}{sqrt{2}}+tfrac{i}{sqrt{2}}right) = boxed{cosh tfrac{1}{sqrt{2}} cos tfrac{1}{sqrt{2}} + isinh tfrac{1}{sqrt{2}}sin tfrac{1}{sqrt{2}}} approx 0.9584 + 0.4986i.$$






Addendum: The rules for handling $f(-t)$ for trig functions $f$ have analogs for handling expressions like $f(it)$; moreover, the same rules apply to hyperbolic and trig functions alike. Let's explore them briefly.

We will distinguish between "c" and "s" functions. The familiar rules are:




  • A factor of $-1$ can be pulled out of c-functions and disappears

  • A factor of $-1$ can be pulled out of s-functions and remains


So:
$cos(-t)=cos t$, and $cosh(-t)=cosh t$;
$sin(-t)=-sin t$, and $sinh(-t)=-sinh t$.



The new rules are only slightly different:




  • A factor of $i$ can be pulled out of c-functions and disappears, transforming the function

  • A factor of $i$ can be pulled out of s-functions and remains, transforming the function


(here, "transforming" means changing trig to hyperbolic and vice versa).



So:
$cos(it)=cosh t$, and $cosh(it)=cos t$; $sin(it)=isinh t$, and $sinh(it)=isin t$.



Now it is easy to see what we said above:



$$cosh(a+bi)=cosh acosh(bi) + sinh asinh(bi)$$
$$= (cosh a)( cos b) + (sinh a) (isin b)$$
$$= cosh a cos b + isinh asin b$$
as desired.






share|cite|improve this answer











$endgroup$





















    0












    $begingroup$

    You can use the fact that:
    $$cosh(x)=frac{e^x+e^{-x}}{2}$$
    And
    $$cos(x)=frac{e^{ix}+e^{-ix}}{2}$$
    Also make use of compound angle formula






    share|cite|improve this answer









    $endgroup$





















      0












      $begingroup$

      Use the definition $cosh(x) = frac{1}{2}(e^{x}+e^{-x})$ and the identity $cosh(x+y) = cosh(x)cosh(y)+sinh(x)cosh(y)$ so



      $$cosh(sqrt{i}) = cosh(sqrt{1/2})cosh(isqrt{1/2}) + sinh(sqrt{1/2})sinh(isqrt{1/2})$$
      $$= cosh(sqrt{1/2})cos(sqrt{1/2}) + isinh(sqrt{1/2})sin(sqrt{1/2})$$






      share|cite|improve this answer









      $endgroup$














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        3 Answers
        3






        active

        oldest

        votes








        3 Answers
        3






        active

        oldest

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        active

        oldest

        votes






        active

        oldest

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        1












        $begingroup$

        Note that there are two possible values for $sqrt{i}$, namely, $pmleft(frac{1}{sqrt{2}}+frac{i}{sqrt{2}}right)$. But since $cosh$ is an even function, it doesn't matter which we choose. So we will work with finding $coshleft(frac{1}{sqrt{2}}+frac{i}{sqrt{2}}right)$.



        One easily verifies that $cosh(a+bi)=cosh a cos b + isinh asin b$. Thus



        $$coshleft(tfrac{1}{sqrt{2}}+tfrac{i}{sqrt{2}}right) = boxed{cosh tfrac{1}{sqrt{2}} cos tfrac{1}{sqrt{2}} + isinh tfrac{1}{sqrt{2}}sin tfrac{1}{sqrt{2}}} approx 0.9584 + 0.4986i.$$






        Addendum: The rules for handling $f(-t)$ for trig functions $f$ have analogs for handling expressions like $f(it)$; moreover, the same rules apply to hyperbolic and trig functions alike. Let's explore them briefly.

        We will distinguish between "c" and "s" functions. The familiar rules are:




        • A factor of $-1$ can be pulled out of c-functions and disappears

        • A factor of $-1$ can be pulled out of s-functions and remains


        So:
        $cos(-t)=cos t$, and $cosh(-t)=cosh t$;
        $sin(-t)=-sin t$, and $sinh(-t)=-sinh t$.



        The new rules are only slightly different:




        • A factor of $i$ can be pulled out of c-functions and disappears, transforming the function

        • A factor of $i$ can be pulled out of s-functions and remains, transforming the function


        (here, "transforming" means changing trig to hyperbolic and vice versa).



        So:
        $cos(it)=cosh t$, and $cosh(it)=cos t$; $sin(it)=isinh t$, and $sinh(it)=isin t$.



        Now it is easy to see what we said above:



        $$cosh(a+bi)=cosh acosh(bi) + sinh asinh(bi)$$
        $$= (cosh a)( cos b) + (sinh a) (isin b)$$
        $$= cosh a cos b + isinh asin b$$
        as desired.






        share|cite|improve this answer











        $endgroup$


















          1












          $begingroup$

          Note that there are two possible values for $sqrt{i}$, namely, $pmleft(frac{1}{sqrt{2}}+frac{i}{sqrt{2}}right)$. But since $cosh$ is an even function, it doesn't matter which we choose. So we will work with finding $coshleft(frac{1}{sqrt{2}}+frac{i}{sqrt{2}}right)$.



          One easily verifies that $cosh(a+bi)=cosh a cos b + isinh asin b$. Thus



          $$coshleft(tfrac{1}{sqrt{2}}+tfrac{i}{sqrt{2}}right) = boxed{cosh tfrac{1}{sqrt{2}} cos tfrac{1}{sqrt{2}} + isinh tfrac{1}{sqrt{2}}sin tfrac{1}{sqrt{2}}} approx 0.9584 + 0.4986i.$$






          Addendum: The rules for handling $f(-t)$ for trig functions $f$ have analogs for handling expressions like $f(it)$; moreover, the same rules apply to hyperbolic and trig functions alike. Let's explore them briefly.

          We will distinguish between "c" and "s" functions. The familiar rules are:




          • A factor of $-1$ can be pulled out of c-functions and disappears

          • A factor of $-1$ can be pulled out of s-functions and remains


          So:
          $cos(-t)=cos t$, and $cosh(-t)=cosh t$;
          $sin(-t)=-sin t$, and $sinh(-t)=-sinh t$.



          The new rules are only slightly different:




          • A factor of $i$ can be pulled out of c-functions and disappears, transforming the function

          • A factor of $i$ can be pulled out of s-functions and remains, transforming the function


          (here, "transforming" means changing trig to hyperbolic and vice versa).



          So:
          $cos(it)=cosh t$, and $cosh(it)=cos t$; $sin(it)=isinh t$, and $sinh(it)=isin t$.



          Now it is easy to see what we said above:



          $$cosh(a+bi)=cosh acosh(bi) + sinh asinh(bi)$$
          $$= (cosh a)( cos b) + (sinh a) (isin b)$$
          $$= cosh a cos b + isinh asin b$$
          as desired.






          share|cite|improve this answer











          $endgroup$
















            1












            1








            1





            $begingroup$

            Note that there are two possible values for $sqrt{i}$, namely, $pmleft(frac{1}{sqrt{2}}+frac{i}{sqrt{2}}right)$. But since $cosh$ is an even function, it doesn't matter which we choose. So we will work with finding $coshleft(frac{1}{sqrt{2}}+frac{i}{sqrt{2}}right)$.



            One easily verifies that $cosh(a+bi)=cosh a cos b + isinh asin b$. Thus



            $$coshleft(tfrac{1}{sqrt{2}}+tfrac{i}{sqrt{2}}right) = boxed{cosh tfrac{1}{sqrt{2}} cos tfrac{1}{sqrt{2}} + isinh tfrac{1}{sqrt{2}}sin tfrac{1}{sqrt{2}}} approx 0.9584 + 0.4986i.$$






            Addendum: The rules for handling $f(-t)$ for trig functions $f$ have analogs for handling expressions like $f(it)$; moreover, the same rules apply to hyperbolic and trig functions alike. Let's explore them briefly.

            We will distinguish between "c" and "s" functions. The familiar rules are:




            • A factor of $-1$ can be pulled out of c-functions and disappears

            • A factor of $-1$ can be pulled out of s-functions and remains


            So:
            $cos(-t)=cos t$, and $cosh(-t)=cosh t$;
            $sin(-t)=-sin t$, and $sinh(-t)=-sinh t$.



            The new rules are only slightly different:




            • A factor of $i$ can be pulled out of c-functions and disappears, transforming the function

            • A factor of $i$ can be pulled out of s-functions and remains, transforming the function


            (here, "transforming" means changing trig to hyperbolic and vice versa).



            So:
            $cos(it)=cosh t$, and $cosh(it)=cos t$; $sin(it)=isinh t$, and $sinh(it)=isin t$.



            Now it is easy to see what we said above:



            $$cosh(a+bi)=cosh acosh(bi) + sinh asinh(bi)$$
            $$= (cosh a)( cos b) + (sinh a) (isin b)$$
            $$= cosh a cos b + isinh asin b$$
            as desired.






            share|cite|improve this answer











            $endgroup$



            Note that there are two possible values for $sqrt{i}$, namely, $pmleft(frac{1}{sqrt{2}}+frac{i}{sqrt{2}}right)$. But since $cosh$ is an even function, it doesn't matter which we choose. So we will work with finding $coshleft(frac{1}{sqrt{2}}+frac{i}{sqrt{2}}right)$.



            One easily verifies that $cosh(a+bi)=cosh a cos b + isinh asin b$. Thus



            $$coshleft(tfrac{1}{sqrt{2}}+tfrac{i}{sqrt{2}}right) = boxed{cosh tfrac{1}{sqrt{2}} cos tfrac{1}{sqrt{2}} + isinh tfrac{1}{sqrt{2}}sin tfrac{1}{sqrt{2}}} approx 0.9584 + 0.4986i.$$






            Addendum: The rules for handling $f(-t)$ for trig functions $f$ have analogs for handling expressions like $f(it)$; moreover, the same rules apply to hyperbolic and trig functions alike. Let's explore them briefly.

            We will distinguish between "c" and "s" functions. The familiar rules are:




            • A factor of $-1$ can be pulled out of c-functions and disappears

            • A factor of $-1$ can be pulled out of s-functions and remains


            So:
            $cos(-t)=cos t$, and $cosh(-t)=cosh t$;
            $sin(-t)=-sin t$, and $sinh(-t)=-sinh t$.



            The new rules are only slightly different:




            • A factor of $i$ can be pulled out of c-functions and disappears, transforming the function

            • A factor of $i$ can be pulled out of s-functions and remains, transforming the function


            (here, "transforming" means changing trig to hyperbolic and vice versa).



            So:
            $cos(it)=cosh t$, and $cosh(it)=cos t$; $sin(it)=isinh t$, and $sinh(it)=isin t$.



            Now it is easy to see what we said above:



            $$cosh(a+bi)=cosh acosh(bi) + sinh asinh(bi)$$
            $$= (cosh a)( cos b) + (sinh a) (isin b)$$
            $$= cosh a cos b + isinh asin b$$
            as desired.







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited Jan 30 at 17:17

























            answered Jan 30 at 16:23









            MPWMPW

            31k12157




            31k12157























                0












                $begingroup$

                You can use the fact that:
                $$cosh(x)=frac{e^x+e^{-x}}{2}$$
                And
                $$cos(x)=frac{e^{ix}+e^{-ix}}{2}$$
                Also make use of compound angle formula






                share|cite|improve this answer









                $endgroup$


















                  0












                  $begingroup$

                  You can use the fact that:
                  $$cosh(x)=frac{e^x+e^{-x}}{2}$$
                  And
                  $$cos(x)=frac{e^{ix}+e^{-ix}}{2}$$
                  Also make use of compound angle formula






                  share|cite|improve this answer









                  $endgroup$
















                    0












                    0








                    0





                    $begingroup$

                    You can use the fact that:
                    $$cosh(x)=frac{e^x+e^{-x}}{2}$$
                    And
                    $$cos(x)=frac{e^{ix}+e^{-ix}}{2}$$
                    Also make use of compound angle formula






                    share|cite|improve this answer









                    $endgroup$



                    You can use the fact that:
                    $$cosh(x)=frac{e^x+e^{-x}}{2}$$
                    And
                    $$cos(x)=frac{e^{ix}+e^{-ix}}{2}$$
                    Also make use of compound angle formula







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Jan 30 at 16:12









                    Henry LeeHenry Lee

                    2,201319




                    2,201319























                        0












                        $begingroup$

                        Use the definition $cosh(x) = frac{1}{2}(e^{x}+e^{-x})$ and the identity $cosh(x+y) = cosh(x)cosh(y)+sinh(x)cosh(y)$ so



                        $$cosh(sqrt{i}) = cosh(sqrt{1/2})cosh(isqrt{1/2}) + sinh(sqrt{1/2})sinh(isqrt{1/2})$$
                        $$= cosh(sqrt{1/2})cos(sqrt{1/2}) + isinh(sqrt{1/2})sin(sqrt{1/2})$$






                        share|cite|improve this answer









                        $endgroup$


















                          0












                          $begingroup$

                          Use the definition $cosh(x) = frac{1}{2}(e^{x}+e^{-x})$ and the identity $cosh(x+y) = cosh(x)cosh(y)+sinh(x)cosh(y)$ so



                          $$cosh(sqrt{i}) = cosh(sqrt{1/2})cosh(isqrt{1/2}) + sinh(sqrt{1/2})sinh(isqrt{1/2})$$
                          $$= cosh(sqrt{1/2})cos(sqrt{1/2}) + isinh(sqrt{1/2})sin(sqrt{1/2})$$






                          share|cite|improve this answer









                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            Use the definition $cosh(x) = frac{1}{2}(e^{x}+e^{-x})$ and the identity $cosh(x+y) = cosh(x)cosh(y)+sinh(x)cosh(y)$ so



                            $$cosh(sqrt{i}) = cosh(sqrt{1/2})cosh(isqrt{1/2}) + sinh(sqrt{1/2})sinh(isqrt{1/2})$$
                            $$= cosh(sqrt{1/2})cos(sqrt{1/2}) + isinh(sqrt{1/2})sin(sqrt{1/2})$$






                            share|cite|improve this answer









                            $endgroup$



                            Use the definition $cosh(x) = frac{1}{2}(e^{x}+e^{-x})$ and the identity $cosh(x+y) = cosh(x)cosh(y)+sinh(x)cosh(y)$ so



                            $$cosh(sqrt{i}) = cosh(sqrt{1/2})cosh(isqrt{1/2}) + sinh(sqrt{1/2})sinh(isqrt{1/2})$$
                            $$= cosh(sqrt{1/2})cos(sqrt{1/2}) + isinh(sqrt{1/2})sin(sqrt{1/2})$$







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Jan 30 at 16:17









                            Nicholas ParrisNicholas Parris

                            1563




                            1563






























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