Вычислите cos 22π/3*cos 13π/6

Question

Вычислите cos 22π/3*cos 13π/6

Answer 1

Cos(22π/3).cos(13π/6)=cos(6π+4π/3).cos(6π+π/6)=
=cos(4π/3).cos(π/6)=-cos(π/3).cos(π/6)=-1/2 . √3/2=-(√3)/4
Otvet: -(√3)/4