Question

1.cos31°=a,则sin239°×tan149°=_________

Answer 1

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