If you click on the "Delete completed" link at the bottom of a task list in a project, it removes all completed tasks within this project on the "Get completed tasks" list at the bottom.
This does not affect the "# Completed" count in the top-right corner which is a full history of your productivity, but you can delete single events on that list manually.
valeu pela atenção David,
agora eu compreendo melhor...
realmente muito obrigado...