cosπ/5 *cos2π/5 的值是？
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利用二倍角公式：sin2θ=2sinθcosθ
cos(π/5)*cos(2π/5)
=2sin(π/5)*cos(π/5)*cos(2π/5)/2sin(π/5)
=sin(2π/5)*cos(2π/5)/2sin(π/5)
=sin(4π/5)/4sin(π/5)
=sin(π/5)/4sin(π/5)
=1/4
如果不懂，请Hi我，祝学习愉快！
cos(π/5)*cos(2π/5)
=2sin(π/5)*cos(π/5)*cos(2π/5)/2sin(π/5)
=sin(2π/5)*cos(2π/5)/2sin(π/5)
=sin(4π/5)/4sin(π/5)
=sin(π/5)/4sin(π/5)
=1/4
如果不懂，请Hi我，祝学习愉快！
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