$$\eqalign{
& f\left( x \right) = y = \cos \left( {x + 3\pi } \right) = \cos \left( {x + \pi } \right) = - \cos x \cr
& TXD:\,\,D = R \Rightarrow \forall x \in D \Rightarrow - x \in D \cr
& f\left( { - x} \right) = - \cos \left( { - x} \right) = - \cos x = f\left( { - x} \right) \cr
& \Rightarrow Ham\,so\,la\,ham\,chan. \cr} $$