Sin a /1 — cos a минус 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