原式=(1+tanatana/2)*2sinacosa/2cosa
=(1+tanatana/2)sina
=sina+tanasinatan(a/2)
=sina+tana*2sin(a/2)*cos(a/2)*sin(a/2)/cos(a/2)
=sina+2tanasin²(a/2)
=sina+2tana(1-cosa)/2
=sina+tana(1-cosa)
=sina+tana-cosa*tana
=sina+tana-cosa*sina/cosa
=sina+tana-sina
=tana