easy
<script>
jQuery.noConflict()(document).ready(function() {
$('#hide').css('display','none');
$('#plano').change(function(){
if(document.getElementById('plano').value == 1){
$('#hide').show('slow');
}else
if(document.getElementById('plano').value == 0){
$('#hide').hide('slow');
}else
if(document.getElementById('plano').value == 0){
$('#hide').css('display','none');
}
});
$('#plano').change();
});
</script>
this example shows and hides the div if selected in combobox some specific value