- #1

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The solution is :

cos z = (e

^{iz}+e

^{-iz})/2 = 0

<=>e

^{iz}+e

^{-iz})=0

<=>e

^{2iz}=-1

<=>2iz=πi +2kπi

<=>z=π/2 + kπ

But i dont understand how does these 2 steps work

<=>e

^{2iz}=-1

<=>2iz=πi +2kπi

how can a exp function suddenly become πi +2kπi

Thanks