jQuery Ajax Request inside Ajax Request

Here is an example:

$.ajax({
    type: "post",
    url: "ajax/example.php",
    data: 'page=" + btn_page,
    success: function (data) {
        var a = data; // This line shows error.
        $.ajax({
            type: "post",
            url: "example.php",
            data: "page=" + a,
            success: function (data) {
   
            }
        });
    }
});