z=tanθ-i
=-cot(π/2+θ)-i
=-1/sin(π/2+θ)(cos(π/2+θ）+isin（π/2+θ))
=1/sin(π/2+θ)(cos(π+π/2+θ）+isin（π+π/2+θ))
=1/sin(π/2+θ)(cos(θ-π/2）+isin（θ-π/2))π/2<θ<π有用嗎？有用，要保證：θ-π/2在0到2π之間。