tana/2
=sin(a/2)/cos(a/2)
=2sin(a/2)cos(a/2)/2cos²(a/2)
=sin2*(a/2)/{2[1+cos2*(a/2)]/2}
=sina/(1+cosa)
tana/2
=2sin²(a/2)/2sin(a/2)cos(a/2)
=2[1-cos2*(a/2)]/sin2*(a/2)
=(1-cosa)/sina