C# htmlagilitypack get element by xpath

<table cellpadding="0" cellspacing="0" id="id_tbl" class=" cke_show_border">
  <tbody>
     <tr>
       <td>
         <span class="title_editor">Heading</span>
       </td>
     </tr>
     <tr>
        <td class="content_tbl" id="id_td">
          <p align="center" style="text-align:center; margin:0in 0in 0.0001pt">
            <span style="font-size:14pt">
              <span style="line-height:150%">
                <span style="font-family:"Times New Roman",serif">
                   < b> hello world < /b>
                </span>
              </span>
             </span>
            </p>
           </td>
          </tr>
      </tbody>
</table>

string curUrl = geckoWebBrowser1.Url.ToString();
                HtmlWeb web = new HtmlWeb();
                HtmlAgilityPack.HtmlDocument doc = web.Load(curUrl);
                var nodes = doc.DocumentNode.SelectNodes("//*[@id='id_td']/p/span/span/span/b/text()");
                richTextBox1.Text = nodes.ToString();