[support] setting.php

My drupal installation is under a subfolder of my domain; therefore, I need to type the subfolder (www.example.com/drupal) on the address bar inorder for drupal to come up. How can I change the setting so I do not need to type the subfolder, like www.example.com instead of www.example.com/drupal? I have tried to change the settings.php $base_url ='http://example.com' instead of $base_url ='http://example.com/drupal', but it still does not work. Any idea? Thanks!