Show that if $X$ has a density $f$ such that $f'$ exists and is integrable, then its characteristic function has the property : $\phi(t)=ο(t^{-1} )$ as $t\to \infty$.
Hint: If $X$ has a density then its characteristic function has the property: $\lim_{t\to \infty}\phi(t)=0$. You may use this result.
how can start with this hint to prove that any help