How to change logo on login page?

Using twenty eleven child theme.

1. how can I change login page logo?
2. added file wp-login.php on child theme with following code, but not controlling the web page. how can I do?
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<?php
/**
* WordPress User Page
*
* Handles authentication, registering, resetting passwords, forgot password,
* and other user handling.
*
* @package WordPress
* @subpackage Twenty_Eleven
* @since Twenty Eleven 1.0
*/
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Please help. thanks!