(cosα)^2=1/(1+(tanα)^2)有没有这个公式,怎么来的?

(cosα)^2=1/(1+(tanα)^2)
有没有这个公式,怎么来的?

α是第三象限角
cosα0
1+(tanα)^2
=1+(sinα)^2/(cosα)^2
=[(cosα)^2+(sinα)^2]/(cosα)^2
=1/(cosα)^2
1/(cosα)^2-1
=[1-(cosα)^2]/(cosα)^2
=(sinα)^2/(cosα)^2
=(tanα)^2
∴原式
1/(cosα×√(1+(tanα)^2)）+2tanα/√(1/(cosα)^2-1）
=1/[cosα×(-1/cosα)]+2tanα/tanα
=-1+2
=1