倍数处理，

<!DOCTYPE html>
<html lang="en">
<head>
   <meta charset="UTF-8">
   <title>倍数处理</title>
</head>
<body>

</body>
<script type="text/javascript">

           var sum = 0;
           var a = 0
           for(i = 1; i <= 99; i++) {
               //i是3的倍数时
               if(i % 3 == 0) {
                   sum += i
                   a = a + i
                   document.write(sum + "<br />")
                   document.write(a)
               }
           }
</script>
</html>