Question

cosa-cosa
1-sina 1+sina

Answer 1

$frac{cosa}{1-sina}-frac{cosa}{1+sina}=frac{cosa(1+sina)-cosa(1-sina)}{(1-sina)(1+sina)}=\\=frac{2sinacdot sina}{1-sin^2a}=frac{2sinacdot cosa}{cos^2a}=frac{2sina}{cosa}=2tga$