(2sinπ/8)(sin3π/8)=2sinπ/8cosπ/8=sinπ/4=（√2）/2(sinπ/12)-(sin5π/12)=sin（π/ 4-π/ 6）-sin（π/ 4+π/ 6）=-2cos(π/4)sin(π/6)=-2*[(√2)/2]*1/2=-(√2)/2原式=-(√2)/2+(√2)/2=0