cos П/5 cos 2П/5=1/4

cosπ/5cos2π/5=(sinπ/5cosπ/5cos2π/5)/sinπ/5==sin2π/5cos2π/5/(2sinπ/5)=sin4π/5/(4sinπ/5)==sin(π-π/5)/(4sinπ/5)=sinπ/5/(4sinπ/5)=1/4