複素数の積 $z_{1} = |z_{1}|(\cos \theta_{1} + i\sin \theta_{1}), z_{2} = |z_{2}|(\cos \theta_{2} + i\sin \theta_{2})$とおく。 \begin{eqnarray*} z_{1}z_{2} &=& |z_{1}||z_{2}| (\cos \theta_{1} + i\sin \theta_{1})(\cos \theta_{2} + i\sin \the…