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