# Solving the complex equation $\sin(z) = \cos(z)$

To find the complex numbers z satisfying $\sin(z) = \cos(z)$, can I say: $$\sin(z) = \frac{(e^{iz}-e^{-iz})}{2i}=\frac{(e^{iz}+e^{-iz})}{2}$$

and solve for z? So we then reduce this to $$-e^{-iz} = e^{-iz}$$ but this doesn't look right

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I don't see how you get from the first equation to the second one. –  celtschk Jul 13 '12 at 6:23

$$\sin(z)=\frac{e^{iz}-e^{-iz}}{2i}$$ Then, $$\sin(z)=\cos(z)\implies \frac{e^{iz}-e^{-iz}}{2i}=\frac{e^{iz}+e^{-iz}}{2}$$ Now let $e^{iz}=t$ ,then $e^{-iz}=1/t$ and you will get a quadratic equation,solve for it and back substitute it to get $z$.

The equation becomes $t^2=\frac{1+i}{1-i}=i=e^{i(\pi/2+2k\pi)}\implies t=e^{i(\pi/4+k\pi)},e^{i(5\pi/4+k\pi)}$. Equating it with $e^{iz}$ we get $z=\pi/4+k\pi$

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so I get 2+i/(2-i) = t^2 and t = +/- sqrt(3/5+4/5i)?? –  mary Jul 13 '12 at 6:03
I think you should get $t^2=\frac{1+i}{1-i}$. –  Aang Jul 13 '12 at 6:10
The last sentence should read: we get $z=\pi/4+k\pi$ for some $k$ in $\mathbb Z$. –  Did Jul 13 '12 at 6:24
right, that would be the general solution. –  Aang Jul 13 '12 at 6:25

The definition of $\sin(z)$ is $$\sin(z)=\frac{e^{iz}-e^{-iz}}{2i}$$

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Use angle addition identities and the relations between trig and hyperbolic trig functions.

\begin{align} &&\sin(z)&=\cos(z)\\ \implies&&\sin(a+bi)&=\cos(a+bi)\\ \implies&&\sin(a)\cos(bi)+\cos(a)\sin(bi)&=\cos(a)\cos(bi)-\sin(a)\sin(bi)\\ \implies&&\sin(a)\cosh(b)+i\cos(a)\sinh(b)&=\cos(a)\cosh(b)-i\sin(a)\sinh(b)\\ \end{align}

Now equating real parts (and keeping in mind that $a$ and $b$ are real), we see that $\sin(a)=\cos(a)$. (This also use the fact that $\cosh(b)$ cannot be $0$.) So $a=\frac{\pi}{4}+k\pi$ for some $k\in\mathbb{Z}$.

Now we can divide across by $\sin(a)$ or $\cos(a)$. (Remember, we have deduced they are equal.)
\begin{align} \implies&&\cosh(b)+i\sinh(b)&=\cosh(b)-i\sinh(b)\\ \implies&&i\sinh(b)&=-i\sinh(b)\\ \implies&&\sinh(b)&=-\sinh(b)\\ \implies&&\sinh(b)&=0\\ \end{align} There is only one real solution for $b$: $b=0$.

So in conclusion, $z=\frac{\pi}{4}+k\pi$ for some $k\in\mathbb{Z}$.

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This to me is the neatest route. +1, of course. –  Guess who it is. Jul 13 '12 at 7:43