Определить угол между двумя плоскими зеркалами, расположенными в одной плоскости, если точечный источник света и два его изображения

1. he is an actor since he has arrived here
2. i was in Germany in 2001 или I have been to Germany        (без даты)
3. We have studied (have been studying) - (если учили present perfect continious) English for three years
4. They have been learning this course for three weeks