cosπ/9cos2π/9cos3π/9cos4π/9=

cosπ/9cos2π/9cos3π/9cos4π/9=
cosπ/9cos2π/9cos3π/9cos4π/9=

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