tan(3a/2)-tan(a/2)
=sin(3a/2)/cos(3a/2)-sin(a/2)/cos(a/2)
=[sin(3a/2)cos(a/2)-sin(a/2)cos(3a/2)]/[cos(3a/2)*cosa]
=sina/[cos(3a/2)*cos(a/2)]
=2sin(a/2)cos(a/2)/[cos(3a/2)*cos(a/2)]
=【2sin（a/2）】/【cos（3a/2）】
得证