cosπ/3＝1/2cosπ/5cos2/π5＝1/4cosπ/7cos2π/7cos3π/7＝1/8cosπ/(2n+1)cos2π/（2n+1)cos3π/(2n+1)------cosnπ/(2n+1)＝1/2^n祝你好运
