cosπ/3＝1/2
cosπ/5cos2/π5＝1/4
cosπ/7cos2π/7cos3π/7＝1/8
cosπ/(2n+1)cos2π/（2n+1)cos3π/(2n+1)------cosnπ/(2n+1)＝1/2^n
已知cosπ/3=1/2,cosπ/5*cos2π/5=1/4,cosπ/7*cos2π/7*cos3π/7=1/8……
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