1-2cos^2a/sina*cosa=tga-ctga
-cos2a/0,5sin2a=-2cos2a/sin2a=-2ctg2a sina/cosa-cosa/sina=(sin²a-cos²a)/sinacosa=-cos2a/0,5sin2a=-2cos2a/sin2a=-2ctg2a -2ctg2a=-2ctg2a