已知tan(a/2)=-2,
则cosa-2sina=[cos^2a/2-sin^2a/2-2*2sina/2cosa/2]/(cos^2a/2+sin^2a/2)
=(1-tan^2a/2-4tana/2)/(1+tan^2a/2),(上下同除以cos^2a/2)
=(1-4+8)/(1+4)
=1