Помогите по алгебре))))))))))

Question

Помогите по алгебре))))))))))

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Answer 1

...=-сos(3п/4+7п/4)=-сos(10п/4)=-сos(п/2)=0
...=сos(70-10)=cоs60=0,5
...=сos(2п/5+3п/5)/сos(п/8+7п/8)=сos(5п/5)/сosп=сosп/сosп=1