http = getHTTPObject();
 
function getHTTPObject(){
  var xmlhttp;
 
  /*@cc_on
 
  @if (@_jscript_version >= 5)
    try {
      xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
    }catch(e){
      try{
      xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }catch(E){
      xmlhttp = false;
    }
  }
  @else
    xmlhttp = false;
  @end @*/
 
  if(!xmlhttp && typeof XMLHttpRequest != 'undefined'){
    try {
      xmlhttp = new XMLHttpRequest();
    }catch(e){
      xmlhttp = false;
    }
  }
 
  return xmlhttp;
}
 
function doMath(){
  var url = "backend.php?op=" + document.getElementById('op').value;
  url += "&num1=" + document.getElementById('num1').value;
  url += "&num2=" + document.getElementById('num2').value;
 
  http.open("GET", url, true);
  http.onreadystatechange = handleHttpResponse;
 
  http.send(null);
}
 
function handleHttpResponse(){
  if(http.readyState == 4){
    document.getElementById('answer').innerHTML = http.responseText;
  }
}