\(\eqalign{
& y = f\left( x \right) = \sin x + \cot x\,\,\left( {D = R\backslash \left\{ {k\pi } \right\}} \right) \cr
& \Rightarrow \forall x \in D \Rightarrow - x \in D \cr
& f\left( { - x} \right) = \sin \left( { - x} \right) + \cot \left( { - x} \right) = - \sin x - \cot x = - f\left( x \right) \cr} \)
=> Hàm số là hàm lẻ.