1.f(sinx)=cos19x,
则f(cosx)=f[sin(π/2-x)]=cos19(π/2-x)=cos(19π/2-19x)=cos(3π/2-19x)=-sin19x.
2.tanα=3,
则cos(π-α)/cos(α-π/2)=-cosα/sinα=-1/tanα=-1/3.
3.θ∈（0,π/2),则θ-sinθ>0,θ-tanθ