(-i)^(1/3)
=(e^(-iπ/2))^(1/3)
=e^(-iπ/6)
=√3/2-i/2

なんだけど、これって

(-i)^(1/3)
=(e^(3iπ/2))^(1/3)
=e^(iπ/2)
=i

ってやっちゃいけないのはなんで？