Please answer the question No.75?

cos(π/9)[sin(π/9)-sin(2π/9)-sin(3π/9)+sin(4π/9)]
  = (sin(2π/9))/2-(sin(3π/9)+sin(π/9))/2
       -(sin(4π/9)+sin(2π/9))/2 + (sin(5π/9)+sin(3π/9))/2
  = -(sin(π/9))/2 - (sin(4π/9))/2 + (sin(5π/9))/2
  = -(sin(π/9))/2   (∵ sin(4π/9) = sin(π-5π/9) = sin(5π/9))

∴  sin(π/9)-sin(2π/9)-sin(3π/9)+sin(4π/9)
       = [-(sin(π/9))/2]/cos(π/9)
       = (-1/2)tan(π/9)