设u=a+π/6，2a+π/12=2u-π/4,cosu=-1/3,sinu=土2√2/3，sin2u=2sinucosu=干4√2/9，cos2u=2(cosu)^2-1=-7/9,sin(2a+π/12)=sin(2u-π/4)=(sin2u-cos2u)/√2=（7√2干8）/18.
