Найти: cos (-780 градусов)

Cos (-ф) = cos (ф)

cos (-780) = cos (780)

cos (a+b) = cos (a) * cos (b) - sin (a) * sin (b)

cos (720 + 60) = cos (720) * cos (60) - sin (720) * sin (60) = 1*cos (60) - 0 * sin (60) = 1*0.5 = 0.5

Cos (-780') = cos (780') = cos (780-360*2) = cos60'=1/2