=sin(π/18)sin(5π/18)sin(－65π/18)
=sin(π/18)sin(5π/18)sin(11π/18)
=sin(π/18)sin(5π/18)sin(7π/18)
=sin(π/18)cos(2π/9)cos(π/9)
=[8cos(π/18)×sin(π/18)×cos(π/9)×cos(2π/9)]/[8cos(π/18)]
=[4sin(π/9)×cos(π/9)×cos(2π/9)]/[8cos(π/18)]
=[sin(4π/9)]/[8cos(π/18)]
=[sin(4π/9)]/[8sin(4π/9)]
=1/8