tg 2x = sin 2x/cos 2x
tg 2x - sin 2x = sin 2x/cos 2x - sin 2x = (sin 2x - sin 2x * cos 2x)/cos 2x = sin 2x * (1 - cos 2x)/cos 2x = sin 2x * (1 - (1 - 2 * sin^2 x))/cos 2x =
= 2 * sin 2x * sin^2 x/cos 2x