<input type="text" name="text1" id="text1">
<input type="text" name="text2" id="text1">
<input type="text" name="text3" id="text1">
<script language="javascript">
document.getElementById('text3').value=parseInt(document.getElementById('text1').value)+parseInt(document.getElementById('text2').value);
</script>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
<title> </title>
<script type="text/javascript">
function add(){
document.getElementById('text3').value=parseInt(document.getElementById('text1').value)+parseInt(document.getElementById('text2').value);
}
</script>
</head>
<body>
<input type="text" name="text1" id="text1">
<input type="text" name="text2" id="text2">
<input type="text" name="text3" id="text3">
<input type="button" name="btn1" id="btn1" value=" add " onclick="add()">
</body>
</html>