z^4=i=cosπ/2+isinπ/2
z=cos[(2nπ+π/2)/4]+isin[(2nπ+π/2)/4]
所以
z=cosπ/8+isinπ/8
z=cos5π/8+isin5π/8
z=cos9π/8+isin9π/8
z=cos13π/8+isin13π/8