cos2π\7乘以COS4π\7乘以COS6π\7

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cos2π\7乘以COS4π\7乘以COS6π\7
cos2π\7乘以COS4π\7乘以COS6π\7

cos2π\7乘以COS4π\7乘以COS6π\7
原式=sinπ/7*cos2π/7*cos4π/7*-cosπ/7/sinπ/7
=-1/2sin2π/7*cos2π/7*cos4π/7/sinπ/7
=-1/4sin4π/7*cos4π/7/sinπ/7
=-1/8sin8π/7/sinπ/7
=1/8sinπ/7/sinπ/7
=1/8