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