In this work we study the existence of new periodic solutions for
the well knwon class of Duffing differential equation of the form
$x^{\prime\prime}+ c x^{\prime}+ a(t) x +b(t) x^3 = h(t)$, where $c$
is a real parameter, $a(t)$, $b(t)$ and $h(t)$ are continuous
$T$--periodic functions. Our results are proved using three
different results on the averaging theory of first order.
