cosπ/9*cos2π/9*cos4π/9怎麼算的?

cosπ/9*cos2π/9*cos4π/9怎麼算的?

分子分母同乘以8sin(π/9),然後分子連續使用二倍角公式====>>>> 2sinacosa=sin2a
===>>>>>原式=[sin(8π/9)]/[8sin(π/9)]=1/8