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\begin{eqnarray*} &&\cos z =\frac{e^{iz}+e^{-iz}}{2}= 0\\\\ \Leftrightarrow&& e^{2iz}=-1=e^{(\pi+2n\pi)i}\\\\ \Leftrightarrow&& z=n\pi + \frac{\pi}{2}\quad(n=0,\pm 1,\pm 2,\cdots) \end{eqnarray*}

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