Cosπ/5*cos2π/5помогите пожалуйста!!!

Question

Cosπ/5*cos2π/5
помогите пожалуйста!!!

Answer 1

Cos(π/5)*cos(2π/5)=sin(π/5)*cos(π/5)*cos(2π/5)/sin(π/5)=sin(2π/5)*cos(2π/5)/2sin(π/5)=sin(4π/5)/4sin(π/5)=1/4