cos(2π/15)+cos(4π/15)+cos(8π/15)+cos(16π/15)
=2cos(9π/15)cos(7π/15)+2cos(6π/15)cos(6π/15)cos(2π/15)
=2cos(3π/5)*[cos(7π/15)-cos(2π/15)]
=-4cos(3π/5)sin(3π/10)sin(π/6)
=-2cos(3π/5)sin(3π/10)