Question

Найти: cos(-780градусов)

Answer 1

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

Answer 2

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