cosπ/15×cos2π/15×cos4π/15×cos8π/15=（sinπ/15×cosπ/15×cos2π/15×cos4π/15×cos8π/15）/（sinπ/15）=（1/2sin2π/15×cos2π/15×cos4π/15×cos8π/15）/（sinπ/15）=（1/4sin4π/15×cos4π/15...