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