Question

сos2pi/5*cos2pi/3*cospi*cos4pi/3*cos5pi/3

Answer 1

      cos2π/5 ·cos2π/3 ·cosπ·cos4π/3·cos5π/3 = 
  = cos72 ·cos(π-π/3) ·(-1)·cos(π+π/3)·cos(2π-π/3)= 
  = - cos72· (-cosπ/3)·(-cosπ/3)·cosπ/3 = -1/8· cos72 =
  = -1/8 ·[(√5-1)/4] = (1-√5)/32