cosπ/9·cos2π/9·cos3π/9·cos4π/9
=(2sinπ/9cosπ/9·cos2π/9·cosπ/3·cos4π/9)/(2sinπ/9)
=(2sin2π/9·cos2π/9·(1/2)·cos4π/9)/(4sinπ/9)
=((1/2)2sin4π/9·cos4π/9)/(8sinπ/9)
=((1/2)sin8π/9)/(8sinπ/9)
=1/16