sin2α=sin(α+α)
=(sinαcosα+cosαsinα)/ 1
=(2sinαcosα)/sin^2α+cos^2α (同除cos^2α)
=2tanα/(1+tan^2α)
tan^2α表示 （tanα）²
cos2α=cos^2 α-sin^2α=(cos^2 α-sin^2α)/(sin^2α+cos^2α)
（同时除以cos^2α)
=(1-tan^2α)/(1+tan^2α)