解sinπ/12cos5π/12=sinπ/12cos(6π/12-π/12)=sinπ/12cos(π/2-π/12)=sinπ/12sinπ/12=sin²π/12=1/2*2sin²π/12=-1/2(1-2sin²π/12)+1/2=-1/2cosπ/6+1/2=-1/2*√3/2+1/2=1/2-√3/4