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;
}
}