（二倍角的三角函数）cosπ/5cos2/5π的值是______cosπ/5cos2π/5 =sinπ/5cosπ/5cos2π/5/sinπ/5 =4sinπ/5cosπ/5cos2π/5/4sinπ/5 =2sin2π/5cos2π/5/4sinπ/5 =sin4π/5/4sinπ/5 =sin(π-π/5)/4sinπ/5 =(1/4)sinπ/5/sinπ/5 =1/4但从4sinπ
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