Решить уравнение:cosx+cos3x=0

Придумайте и решите 10 примеров на сложение и вычитание целых чисел!Заранее спасибо!:)

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 My favourite sport is tennis. I don't play tennis but I like to watch how they play. When I am going  to be older I will be a good player. I think tennis is the best sport! It is amazing how much strength they use. Tennis is a hard sport like swimming. So, that's all I can tell for now about my favourite sport.