insert,update, delete derived entity in entity framework 4.0

Question

Hi! How do i insert an entity that is derived from another entity. here is my code but it's not working:(applicationreplacement derived from application

public void AddReplacementApp(Application entity,ApplicationReplacement rentity)
{            
     _ctx.CreateObjectSet<Application>().AddObject(rentity);
     _ctx.SaveChanges();    
}