JavaScript使用嵌套循环生成梯形乘法表

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
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<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<script type="text/javascript">
var str= "<table border=1 width=500 align=center>";
for(var a=1;a<=9;a++)
{  
  str +="<tr>";
  for(b=1;b<=a;b++)
  {
  str +="<td>"+a+"&times;"+b+"="+a*b+"</td>";
  }
  str +="</tr>";
}
str +="</table>"
document.write(str);
</script>
</head>
<body>
</body>
</html>