求大神指点下。点击“=”按钮，没有反应呢?
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<script type="text/javascript">
function count(){
var num1=document.getElementById("num1").value ,
symble=document.getElementById("symble"), num2=document.getElementById("num2").value ,
result;
switch(symble){
case"+":
result=parseInt(num1)+parseInt(num2);
break;
case"-":
result=parseInt(num1)-parseInt(num2);
break;
case"*":
result=parseInt(num1)*parseInt(num2);
break;
case"/":
result=parseInt(num1)/parseInt(num2);
break;
default:
result=null;
}
document.getElementById("num3") = result;
}
</script>>
<title>计算器简单功能实现</title>
</head>
<body>
<input type="text" name="num" id="num1">
<select id="symble">
<option>+</option>
<option>-</option>
<option selected="selected">*</option>
<option>/</option>
</select>
<input type="text" name="num" id="num2">
<input type="button" value="=" onclick = "count()">
<input type="text" name="num" id="num3">
</body>
</html>
