how use serialize in jquery with my customize data?

i have this function:

我有这个功能：

function save_grad_form_one(id,a,b)
{

    $("#loading").ajaxStart(function(){$(this).show();});
    $("#loading").ajaxStop(function(){$(this).hide();});  

    $.ajax({
    type:"POST",
    url:"student/class/ajax/save_grade.php", 
    data:($(":input").serialize(),{id:id ,a_one:a ,b_one:b}),
    success:function(data){
        if((data.result)=='true')
            alert(data.output);
            );
        }, 
    dataType:"json"});
    return false;

}
function save