Tuesday, 18 June 2019

trigonometry - Understanding Euler's Identity



I would like to understand one specific moment in Euler's Identity, namely




ejθ=cos(θ)+jsin(θ)



where j=1. We also know that



ej2(π)=cos(2π)+jsin(2π)



but sin(2π)=0 and cos(2π)=1, and 1=e0, so we get that ej2(π)=e0.



But we get that j2π=0 which means that j=0, but on the other hand j=1. I want to ask one question: why is it allowed to use such symbols in identity, which finally may cause some strange equality?


Answer




The mistake is that exp:CC is no longer a bijection. Thus
e2πi=e0
In general
\exp(z) = \exp(z+2\pi i) \qquad \forall\ z\in\mathbb C
because of the periodicity of \sin and \cos and the definition
\exp(z) = \underbrace{\exp(\Re z)}_{\exp: \mathbb R\to\mathbb R} \cdot (\cos(\Im z) + i\sin(\Im z))


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