getting error while using tab container in asp.net

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VB

ASP.NET

Hi all,

This is my source code:

<form id="form1" runat="server">
   <div>
   <asp:ScriptManager ID="ScriptManager1" runat="server">
   </asp:ScriptManager>
   <asp:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="0"
       Height="1191px" style="margin-bottom: 103px" Width="823px" AutoPostBack="True">
       <asp:TabPanel runat="server" HeaderText="TabPanel1" ID="TabPanel1">

           <ContentTemplate>
                              <asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>
                               <asp:TextBox ID="TextBox4" runat="server"></asp:TextBox>
                              <asp:TextBox ID="TextBox5" runat="server"></asp:TextBox>
                               <asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" ></asp:Button>
           </ContentTemplate>
       </asp:TabPanel>
   </asp:TabContainer>
   </div>
   </form>

And my code is:

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs)

      TextBox5.Text = TextBox3.Text + TextBox4.Text
  End Sub

And I am getting "Microsoft JScript runtime error: 'Sys.Extended.UI' is null or not an object" error in 4th line of the below jscript code:

<script type="text/javascript">
//<![CDATA[
Sys.Application.add_init(function() {
$create(Sys.Extended.UI.TabPanel, {"headerTab":$get("__tab_TabContainer1_TabPanel1"),"ownerID":"TabContainer1","wasLoadedOnce":false}, null, {"owner":"TabContainer1"}, $get("TabContainer1_TabPanel1"));});
Sys.Application.add_init(function() {
$create(Sys.Extended.UI.TabContainer, {"activeTabIndex":0,"autoPostBackId":"TabContainer1","clientStateField":$get("TabContainer1_ClientState"),"onDemand":false,"tabStripPlacement":0,"useVerticalStripPlacement":false}, null, null, $get("TabContainer1"));

Can any one help me to overcome this kind of error?
});