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