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Eulerの公式の証明

Last updated at Posted at 2019-05-09

これが一番楽だと思います(多分)

証明

複素数平面における単位円上の複素数$z$は、次のように書ける。

  z  =  \cos \theta  +i \sin \theta

両辺を$\theta$で微分すると、

  \frac{dz}{d \theta}  =  -\sin \theta  +i \cos \theta  =  iz

変数分離すると、

  \frac{1}{z} \frac{dz}{d \theta}  =  i

両辺を$\theta$で積分すると、

  \int \frac{1}{z} \frac{dz}{d \theta} d \theta  =  \int \frac{1}{z} dz  =  \int i d \theta
  \\
  \log_e z  =  i \theta
  \\
  z  =  e^{i \theta}

よって、

  e^{i \theta}  =  \cos \theta  +i \sin \theta

この式を、オイラーの公式という。

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