Sin a/1+cos a + sin a/1-cos a

Sin a / (1 - cos a) - sin a / (1+cos a) = (sin a (1+cos a) - sin a (1-cos a)) / (1 - cos a) (1+cos a) = sin a (1+cos a-1+cos a) / (1 - cos a) (1+cos a) = sin a * cos^2 a/1-cos^2 a=sin a * cos^2 a/sin^2 a=cos^2 a/sin a = ctg^2 a*sin a