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