1.$("table tr").find("td:first").html(11);
?
2.$("table tr").each(function(){
$(this).find("td:eq(1)").html(22);?????
})
?
3.$("table tr td:first-child").html("first")；
$("table tr td:last-child").html("last")
?
4.$("table tr td:nth-child(3)").html(33)
$("table tr td:nth-child(odd)").html(44)
$("table tr td:nth-child(even)").html(55)
$("table tr td:nth-child(3n+1)").html(66)