jquery - Web service call with ajax in asp.net -

i trying use web service using jquery ajax. webservice method called , parameter passed well, wonder why ajax not execute success function

here's ajax call

<script type="text/javascript">     function btn_add() {         var = $("#tb_a").val();         var b = $("#tb_b").val();         var c = $("#tb_c").val();         var para = "a=" + + "&b=" + b + "&c=" + c;          $.ajax({             type: "post",             url: "mywebservice.asmx/add",             data: para,             datatype: "text",             success: function (res) {                 alert("success");                 console.log(res);             }         });     } </script> 

edit

here webservice code

 [system.web.script.services.scriptservice]     public class mywebservice : system.web.services.webservice     {          [webmethod, scriptmethod]         public int add( int a, int b, int c)         {             return + b + c;         }     } 

you should take @ documentation, says call like this:

     $.ajax({         type: "post",         url: "mywebservice.asmx/add",         data: { : $("#tb_a").val(),                  b : $("#tb_b").val(),                 c : $("#tb_c").val() },         success: function (res) {             alert("success");             console.log(res);         }     }); 

if in server have this:

public string myfunction(string a, string b, string c) {     return + b + c; } 

now, if doesn't work, take @ answer, it's different way accomplish want