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Homework Help: Help complex equation cos(z) = -isin(z)

  1. Jan 9, 2009 #1
    1. The problem statement, all variables and given/known data

    Let z be a complex number. I want to solve cos(z)= -i*sin(z).

    3. The attempt at a solution

    Here's my work:

    cos(z) = -i*sin(z) implies cos(z) + isin(z) = 0.
    Therefore exp(i*z) = 0. Now put z= x+iy then i*z = i*(x+iy) = ix - y, hence
    exp(i*z) = exp(ix-y) = exp(ix)*exp(y) =0 but exp(y) is always nonzero so this implies
    exp(i*x) = 0 hence cos(x)+i*sin(x) =0 thus cos(x)=0 and sin(x)=0 which is impossible.

    So I think there are no solutions. Is this correct?
  2. jcsd
  3. Jan 9, 2009 #2
    No. Here, 0 is also a complex number (0+i0). Now you have cosz + isinz=0+i0. As the real and imaginary parts must be equal, you have two conditions, cosz=0 and sinz=0. That gives you values for z and the total solution is the union of those two solutions.
  4. Jan 9, 2009 #3

    D H

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    Staff Emeritus
    Science Advisor

    No. First off, you are implicitly assuming that cos z and sin z are real. Secondly, the total solution will be the intersection, not union.

    Carl140: You're analysis is correct.
  5. Jan 10, 2009 #4
    You're right. My mistake. I messed up.
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