Sina/1-cosa - ctga = sina/1-cosa - cosa/sina = sin^2a - cos + cos^2a/sina(1-cosa) = 1-cosa /(1-cosa)*sina = 1/sina
1+cosa - sin^2acosa/1-cosa = 1-cos^2a - sin^2cosa/ 1-cosa = sin^2a-sin^2acosa/ 1+cos^2 = sin^2
Cosa/1-sina - sina/cosa = cos^2-sia + sin^2a/cosa(1-sina) = 1/cosa
1 + sina - cos^2asina/1-sina = cos^2a-cos^2asina/1-sina= cos^2a(1-sina)/(1-sina) = cos^2