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)
cos2/15π+cos4/15π+cos8/15π+cos16/15π
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