Ctg²(α)*sin²(α)-1 ( cos²(α)/sin²(α) ) * sin²(α)- (sin²(α)+cos²(α))
------------------------- = ----------------------------------------------------------------- =
tg( α)*cos(α) (sin(α)/cos(α) ) * cos(α)
-sin²(α)
= ------------------ = - sin(α)
sin(α)