1/cosa-cosa/(1+sina)=cosa/(1-sina^2) -cosa(1-sina)/(1-sina^2)
=[cosa-cosa+sinacosa]/(1-sina^2)
=sinacosa/cosa^2=sina/cosa=tana
1/cosa-cosa/(1+sina)=cosa/(1-sina^2) -cosa(1-sina)/(1-sina^2)
=[cosa-cosa+sinacosa]/(1-sina^2)
=sinacosa/cosa^2=sina/cosa=tana