sin x * ctgx - 2 cos x
sin x * (1/tan x) - 2 cos x
sin x * (cos x/sin x) - 2 cos x
cos x - 2 cos x
= -cos x
sin x * ctgx - 2 cos x
sin x * (1/tan x) - 2 cos x
sin x * (cos x/sin x) - 2 cos x
cos x - 2 cos x
= -cos x