作业帮求值:cos2π/7cos4π/7cos8π/7
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求值:cos2π/7cos4π/7cos8π/7
求值:cos2π/7cos4π/7cos8π/7
求值:cos2π/7cos4π/7cos8π/7
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cos(2π/7)cos(4π/7)cos(8π/7)
=2sin(2π/7)cos(2π/7)cos(4π/7)cos(8π/7)/【2sin(2π/7)】
=sin(4π/7)cos(4π/7)cos(8π/7)/【2sin(2π/7)】
=2sin(4π/7)cos(4π/7)cos(8π/7)/【4sin(2π/7)】
=sin(8π/7)cos(8π/7)...
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cos(2π/7)cos(4π/7)cos(8π/7)
=2sin(2π/7)cos(2π/7)cos(4π/7)cos(8π/7)/【2sin(2π/7)】
=sin(4π/7)cos(4π/7)cos(8π/7)/【2sin(2π/7)】
=2sin(4π/7)cos(4π/7)cos(8π/7)/【4sin(2π/7)】
=sin(8π/7)cos(8π/7)/【4sin(2π/7)】
=2sin(8π/7)cos(8π/7)/【8sin(2π/7)】
=sin(16π/7)/【8sin(2π/7)】
=sin(2π/7)/【8sin(2π/7)】
=1/8
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cosπ/7cos2π/7cos4π/7cos8π/7=?每步都写
化简cosπ/7*cos2π/7*cos4π/7 提示：据分析应该用倍角公式.
cosπ/7cos2π/7cos4π/7=?写下过程,谢谢~
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