AJAX addition

<!DOCTYPE html>
<html>
<head>
    <script>
        function addJS() {
            var a = document.getElementById("aj").value;
            var b = document.getElementById("bj").value;
            document.getElementById('cj').value = Number(a) + Number(b);
        }
        function showSum() {
            var a = Number(document.getElementById("a").value);
            var b = Number(document.getElementById("b").value);

            var xmlhttp=new XMLHttpRequest();
            xmlhttp.onreadystatechange = function() {

                    document.getElementById('c').value=xmlhttp.responseText;
            }
            xmlhttp.open("GET","d1.php?a="+a+"&&b="+b, true);
            xmlhttp.send();
        }
    </script>
</head>
<body>

<form>
    <input type="number" id="a" onchange="showSum()" /> +
    <input type="number" id="b" onchange="showSum()" /> =
    <input type="number" id="c" />
</form>
<form>
    <input type="number" id="aj" /> +
    <input type="number" id="bj" /> =
    <input type="number" id="cj" />
    <input type="button" value="JS" onclick="addJS()" />
</form>
</body>
</html>


<?php echo $_REQUEST["a"]+$_REQUEST["b"]; ?>