π/6π/6
π/6∵π/6
cos(a-π/6)=15/17,因为π/6cosa=cos[(a-π/6)+π/6]=cos(a-π/6)cosπ/6-sin(a-π/6)sinπ/6=15/17*√3/2-8/17*1/2=(15√3-8)/34