Question

Помогите по алгебре))))))))))

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