1－cosA＝2[sin(A/2)]^2
sinA＝2sin(A/2)cos(A/2)
z＝1－cosA＋isinA
＝2sin(A/2)[sin(A/2)＋icos(A/2)]
＝2sin(A/2)[cos(π/2－A/2)＋isin(π/2－A/2)]
|Z|＝2|sin(A/2)|,辐角是π/2－A/2