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