α是第三象限角cosα01+(tanα)^2=1+(sinα)^2/(cosα)^2=[(cosα)^2+(sinα)^2]/(cosα)^2=1/(cosα)^21/(cosα)^2-1=[1-(cosα)^2]/(cosα)^2=(sinα)^2/(cosα)^2=(tanα)^2∴原式1/(cosα×√(1+(tanα)^2)）+2t...