cos兀/7cos2兀/7cos3兀/7=?
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![cos兀/7cos2兀/7cos3兀/7=?](/uploads/image/z/11957357-29-7.jpg?t=cos%E5%85%80%2F7cos2%E5%85%80%2F7cos3%E5%85%80%2F7%3D%3F)
cos兀/7cos2兀/7cos3兀/7=?
cos兀/7cos2兀/7cos3兀/7=?
cos兀/7cos2兀/7cos3兀/7=?
原式=sinπ/7*cosπ/7*cos2π/7*cos3π/7/sinπ/7
=2sinπ/7*cosπ/7*cos2π/7*cos(π-4π/7)/2sinπ/7
=sin2π/7*cos2π/7*(-cos4π/7)/2sinπ/7
=-2sin2π/7*cos2π/7*cos4π/7/4sinπ/7
=-sin4π/7*cos4π/7/4sinπ/7
=-2sin4π/7*cos4π/7/8sinπ/7
=-sin8π/7/8sinπ/7
=sinπ/7/8sinπ/7
=1/8