The given function is tan −1 ( 1−cosx 1+cosx ) , x<π.
Simplify the given function,
tan −1 ( 1−cosx 1+cosx )= tan −1 ( 2 sin 2 x 2 2 cos 2 x 2 ) = tan −1 ( sin x 2 cos x 2 ) = tan −1 ( tan x 2 ) = x 2
Therefore, the simplest form of the given function is x 2 .