cos 780 градусов=cos (720+60 градусов)=......?????

Question

cos 780 градусов=cos (720+60 градусов)=......?????

Answer 1

cos780=cos(720+60)=cos60=1/2

 

Answer 2

т.к. функция cos 2π периодична, то cos720=cos360=cos2π

cos(2π+α)=cosα

cos(360+60)=cos60=1/2