用JS动态改变表单form里的action值属性的方法

<form id="form1" name="form1" method="post" action="../news/index.asp">
      <table width="100%" height="43" border="0" cellpadding="0" cellspacing="0">
        <tr>
          <td height="28"><input name="keyword" type="text" style="width:150px" id="keyword"/></td>
        </tr>
        <tr>
          <td height="28"><select name="Searchtype"  style="width:110px" id="Searchtype" onchange="Searchtype1();">
            <option value="news" selected="selected">新闻中心</option>
            <option value="case">工程案例</option>
          </select>
          <input type="submit" name="Submit" value="搜索" /></td>
        </tr>
      </table>
  </form>
  <script language="javascript">
  function Searchtype1(){
  var type=document.getElementById("Searchtype").options[document.getElementById("Searchtype").selectedIndex].value;
  if (type=="news"){document.getElementById("form1").action="../news/index.asp"}
  else if (type=="case"){document.getElementById("form1").action="../case/index.asp"}
  }
  </script>