cos1/7*cos2/7*cos3/7=?

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cos1/7*cos2/7*cos3/7=?cos1/7*cos2/7*cos3/7=?cos1/7*cos2/7*cos3/7=?cos(1/7pi)*cos(2/7pi)*cos(3/7pi)=-

cos1/7*cos2/7*cos3/7=?
cos1/7*cos2/7*cos3/7=?

cos1/7*cos2/7*cos3/7=?
cos(1/7pi)*cos(2/7pi)*cos(3/7pi)
=-cos(1/7pi)*cos(2/7pi)*cos(4/7pi)
=-4sin(1/7pi)*cos(1/7pi)*cos(2/7pi)*cos(4/7pi)/4sin(1/7pi)
=-2sin(2/7pi)cos(2/7pi)*cos(4/7pi)/4sin(1/7pi)
=-sin(4/7pi)cos(4/7pi)/4sin(1/7pi)
=-1/2sin(8/7pi)/4sin(1/7pi)
=-1/2sin(1/7pi)/4sin(1/7pi)
=-1/8