unable to resolve error i.e 'jQuery' is undefined

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C#

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jQuery

hi friends
i've a piece of code when i view it in browser it gives 'jQuery' is undefined error.

In this code i Want datepicker to be implemented however i'm not able to do so
I've reffered many sites and i'm finding similar kind of code written
please help me out to find out where i'm wrong

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">

    <script src="jscript/jquery.datePicker.js" type="text/javascript"></script>

    <script src="jscript/jquery-1.5.1.min.js" type="text/javascript"></script>

     <script type="text/javascript">
        $(function() {
           $('#startdate').datepicker({ dateFormat: 'dd/mm/yyyy' });
           $('#enddate').datepicker({ dateFormat: 'dd/mm/yyyy' });
          });
    </script>


<div id="content">        From <asp:TextBox ID="startdate" class="field" runat="server"></asp:TextBox> -
 To <asp:TextBox ID="enddate" class="field" runat="server"></asp:TextBox>    </div>
</asp:Content>

i'm using master page
in which i've included these two following references that are used here

<link href="jscript/jquery.ui.all.css" rel="stylesheet" type="text/css" />
   <link href="jscript/datePicker.css" rel="stylesheet" type="text/css" />

i've checked the script with other functions is working for eg $('#startdate').value("some text") works very well
but datepicker method is not working
it is also giving error that this method is not supported by object

Thanks in advance....