求值cosπ/9+cos3π/9+cos5π/9+cos7π/9
问题描述：
求值cosπ/9+cos3π/9+cos5π/9+cos7π/9
答
cosπ/9+cos3π/9+cos5π/9+cos7π/9=cos3π/9+cos(π-π/9)+cos(π-4π/9)+cos(π-2π/9)=cosπ/3-(cos8π/9+cos4π/9+cos2π/9) *** cos(π-α) = -cosα=1/2-(cos(π-π/9)+cos(3π/9+π/9)+cos(3π/9-π/9)) =1/...