原式=2sin(π/5)cos(π/5)cos(2π/5)/[2sin(π/5)]=sin(2π/5)cos(2π/5)/[2sin(π/5)]=2sin(2π/5)cos(2π/5)/[4sin(π/5)]=sin(4π/5)/[4sin(π/5)]=sin(π-π/5)/[4sin(π/5)]=sin(π/5)/[4sin(π/5)]=1/4