请帮我看下最后那个javascript语法，怎么在打开下一个菜单栏的时候收缩上一个打开的菜单栏。。
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="left_stu.aspx.cs" Inherits="left_stu" %>

<title>在线学习考试系统</title>
<td>
<table width="160" height="100%" border="0" cellpadding="0" cellspacing="0">
    <tr>
    <td height="4%" background="images/main_40.gif"><table width="100%" border="0" cellspacing="0" cellpadding="0">
      <tr>
        <td width="19%">&nbsp;</td>
        <td width="81%" height="20"><span class="STYLE1">系统菜单</span></td>
      </tr>
    </table></td>
  </tr>
<tr>
    <td valign="top">
     <table width="151" border="0" align="center" cellpadding="0" cellspacing="0">
      <tr>
        <td>
         <table width="100%" border="0" cellspacing="0" cellpadding="0">
          <tr>
            <td height="23" background="images/main_47.gif" id="imgmenu1" class="menu_title" onMouseOver="this.className='menu_title2';" onClick="showsubmenu(1)" onMouseOut="this.className='menu_title';" style="cursor:hand"><table width="100%" border="0" cellspacing="0" cellpadding="0">
              <tr>
                <td width="18%">&nbsp;</td>
                <td width="82%" class="STYLE1">互动交流</td>
              </tr>
            </table></td>
          </tr>
          <tr>
            <td background="images/main_51.gif" id="submenu1">
 <div class="sec_menu" >  
<table width="100%" border="0" cellspacing="0" cellpadding="0">
              <tr>
                <td><table width="90%" border="0" align="center" cellpadding="0" cellspacing="0">
                  <tr>
                    <td width="16%" height="25"><div align="center"><img src="images/left.gif" width="10" height="10" /></div></td>
                   &n