如何左对齐 右对齐了

设了居中后 无法左对齐 右对齐了
该怎么修改了<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
<style type="text/css">
</style>
</head><body>
<div style="width:1000px;; height:1000px;margin:0 auto;text-align:center;">
<div style="height:30px; width:500px; background-color:#0000FF; color:#FFFFFF;">
<span>左对齐</span>
<span>右对起</span>
</div>
</div>
</body>
</html>