cosπ/12*cos5π/12=cos5π/12*cosπ/12=1/2*[cos(5π/12+π/12)+cos(5π/12-π/12)=1/2*[cosπ/2+cosπ/3]=1/2*[0+1/2]=1/2*1/2=1/4用的是积化和差公式,属于高中要求范围的
