tg55°-tg35°=sin(55°-35°)/(cos55°*cos35°)=sin20°/(cos55°*cos35°)
cos55°*cos35°=[cos(55°+35°)+cos(55°-35°)]/2=(cos90°+cos20°)/2=0/2+cos20°/2=cos20°/2
tg55°-tg35°= sin20°/(cos55°*cos35°)=sin20°/cos20°/2=2sin20°/cos20°=2tg20°
