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                        /形形色色的下拉菜單

你好，我的代碼為什么下拉菜單沒有顯示出來，報錯信息是hideSubMenu

<!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>
????<meta?http-equiv="Content-Type"?content="text/html;?charset=utf-8"?/>
????<title>無標題文檔</title>
????<style?type="text/css">
????????*{?margin:0px;?padding:0px;}
????????#menu{?background-color:#eee;?width:600px;?height:40px;?margin:0?auto;}
????????ul{?list-style:none;}
????????ul?li{?line-height:40px;?text-align:center;?position:relative;float:?left;?}
????????a{?text-decoration:none;?color:#000;?display:block;?width:90px;}
????????a:hover{?color:#FFF;?background-color:#666;}
????????ul?li?ul?li{?float:none;margin-top:2px;?background-color:#eee;?}
????????ul?li?ul{width:90px;?position:absolute;display:?none;?}
????</style>
????<script?type="javascript">
????????function?displaySubMenu(li)?{
????????????var?submenu=li.getElementsByTagName("ul")[0];
????????????submenu.style.display="block";
????????}
????????function?hideSubMenu(li)?{
????????????var?submenu=li.getElementsByTagName("ul")[0];
????????????submenu.style.display="none";
????????}
????</script>
</head>

<body>
<div?id="menu">
????<ul>
????????<li><a?href="#">首頁</a></li>
????????<li?onmouseover="displaySubMenu(this);"?onmouseout="hideSubMenu(this);"><a?href="#">課程大廳</a>
????????????<ul>
????????????????<li><a?href="#">JavaScript</a></li>
????????????????<li><a?href="#">JQuery</a></li>
????????????</ul>
????????</li>
????????<li><a?href="#">學習中心</a>
????????????<ul>
????????????????<li><a?href="#">視頻學習</a></li>
????????????????<li><a?href="#">案例學習</a></li>
????????????????<li><a?href="#">交流平臺</a></li>
????????????</ul>
????????</li>
????????<li><a?href="#">經典案例</a></li>
????????<li><a?href="#">關于我們</a></li>
????</ul>
</div>
</body>
</html>