<h3>示例1</h3>
<div class="navbar navbar-default" id="navmenu">
  <a href="##" class="navbar-brand">W3cplus</a>
  <ul class="nav navbar-nav">
    <li class="dropdown">
      <a href="##" data-toggle="dropdown" class="dropdown-toggle" role="button" id="tutorial">教程<b class="caret"></b></a>
      <ul class="dropdown-menu" role="menu" aria-labelledby="tutorial">
        <li role="presentation"><a href="##">CSS3</a></li>
        <li role="presentation"><a href="##">HTML5</a></li>
        <li role="presentation"><a href="##">Sass</a></li>
      </ul>
    </li>
    <li><a href="##">前端论坛</a></li>
    <li><a href="##">关于我们</a></li>
  </ul>
</div>

<h3>示例2</h3>
<ul class="nav nav-pills">
  <li class="dropdown">
    <a href="##" data-toggle="dropdown" class="dropdown-toggle" role="button" id="tutorial">教程<b class="caret"></b></a>
    <ul class="dropdown-menu" role="menu" aria-labelledby="tutorial">
      <li role="presentation"><a href="##">CSS3</a></li>
      <li role="presentation"><a href="##">HTML5</a></li>
      <li role="presentation"><a href="##">Sass</a></li>
    </ul>
  </li>
  <li class="active"><a href="##">前端论坛</a></li>
  <li><a href="##">关于我们</a></li>
</ul>
<script src="http://libs.baidu.com/jquery/1.9.0/jquery.js"></script>
<script src="http://cdn.bootcss.com/bootstrap/2.3.1/js/bootstrap-dropdown.js"></script>

<div class="dropdown">
  <a data-toggle="dropdown" href="#">下拉菜单触发器</a>
  <ul class="dropdown-menu" role="menu" aria-labelledby="dLabel"> ... </ul>
</div>

<div class="dropdown">
  <a id="dLabel" role="button" data-toggle="dropdown" data-target="#" href="/page.html"> Dropdown <span class="caret"></span></a>
  <ul class="dropdown-menu" role="menu" aria-labelledby="dLabel"> ... </ul>
</div>


<ul class="nav nav-pills">
  <li><a href="##">教程</a></li>
  <li class="active"><a href="##">前端论坛</a></li>
  <li class="dropdown">
    <a href="##" data-toggle="dropdown" class="dropdown-toggle">关于我们</a>   

     <ul class="dropdown-menu" role="menu" aria-labelledby="tutorial">
        <li role="presentation"><a href="##">CSS3</a></li>
        <li role="presentation"><a href="##">HTML5</a></li>
        <li role="presentation"><a href="##">Sass</a></li>
     </ul>
  </li>
</ul>

<ul class="nav nav-pills">
  <li class="dropdown">
    <a href="##" class="dropdown-toggle" role="button" id="tutorial">教程<b class="caret"></b></a>
    <ul class="dropdown-menu" role="menu" aria-labelledby="tutorial">
      <li role="presentation"><a href="##">CSS3</a></li>
      <li role="presentation"><a href="##">HTML5</a></li>
      <li role="presentation"><a href="##">Sass</a></li>
    </ul>
  </li>
  <li class="active"><a href="##">前端论坛</a></li>
  <li><a href="##">关于我们</a></li>
</ul>

<script src="http://libs.baidu.com/jquery/1.9.0/jquery.js"></script>
<script src="http://maxcdn.bootstrapcdn.com/bootstrap/3.2.0/js/bootstrap.min.js"></script>

<script>
  $(function(){
  $(".dropdown-toggle").dropdown();
})  
</script>

//显示下拉菜单：
$(function(){
  $(".dropdown-toggle").dropdown("toggle");
})