cos（π/7）*cos（2π/7）*cos（4π/7）
=[2sin(π/7)cos（π/7）*cos（2π/7）*cos（4π/7）]/2sin(π/7)
=[sin(2π/7)*cos（2π/7）*cos（4π/7）]/2sin(π/7)
=[2sin(2π/7)*cos（2π/7）*cos（4π/7）]/4sin(π/7)
=[sin(4π/7)*cos(4π/7)]/4sin(π/7)
=[2sin(4π/7)*cos(4π/7)]/8sin(π/7)
=sin(8π/7)/8sin(π/7) (因?yàn)閟in(8π/7)=-sin（π/7）)
=-1/8
化簡(jiǎn)cosπ/7*cos2π/7*cos4π/7
化簡(jiǎn)cosπ/7*cos2π/7*cos4π/7
提示：據(jù)分析應(yīng)該用倍角公式.
提示：據(jù)分析應(yīng)該用倍角公式.
數(shù)學(xué)人氣：237 ℃時(shí)間：2020-03-19 08:33:13
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