求值:cosπ/7cos2π/7cos3π/7cos4π/7cos5π/7cos6π/7
问题描述：
求值:cosπ/7cos2π/7cos3π/7cos4π/7cos5π/7cos6π/7
答
由诱导公式可得：cos6π/7=-cosπ/7；cosπ/7=-cos6π/7,cos3π/7=-cos4π/7,cos5π/7=-cos2π/7∴ cos2π/7cos4π/7cos6π/7 =-cosπ/7cos2π/7cos4π/7=-sinπ/7cosπ/7cos2π/7cos4π/7/sinπ/7=-1/2·sin2π/7co...