1.cos31°=a,则sin239°×tan149°=---------

1.sin239°=sin(270°-31°)=-sin(90°-31°)=-cos31°=-a
tan149°=tan(180°-31°)=-sin31°/cos31°
sin239°*tan149°=sin31°=(1-a^2)^0.5
2.cos(2/3*pie-a)=cos(pie/2+pie/6-a)=-sin(pie/6-a)=-m
3.cosx=sin(pie/2-x)
f(cosx)=f(sin(pie/2-x))=cos(pie-2x)=-cos2x