cosπ/17cos2π/17cos4π/17cos8π/17
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cosπ/17cos2π/17cos4π/17cos8π/17
=sinπ/17*cosπ/17*cos2π/17*cos4π/17*cos8π/17 / sinπ/17
=1/2*sin2π/17*cos2π/17*cos4π/17*cos8π/17 / sinπ/17
=1/4sin4π/17*cos4π/17*cos8π/17 / sinπ/17
=1/8*sin8π/17*cos8π/17 / sinπ/17
=1/16*sin16π/17 / sinπ/17
sin16π/17=sinπ/17
所以原式=1/16
=sinπ/17*cosπ/17*cos2π/17*cos4π/17*cos8π/17 / sinπ/17
=1/2*sin2π/17*cos2π/17*cos4π/17*cos8π/17 / sinπ/17
=1/4sin4π/17*cos4π/17*cos8π/17 / sinπ/17
=1/8*sin8π/17*cos8π/17 / sinπ/17
=1/16*sin16π/17 / sinπ/17
sin16π/17=sinπ/17
所以原式=1/16
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