tan(π/4+a/2)=(1+tan(a/2))/(1-tan(a/2))反复利用万能公式tana+1/tan(π/4+a/2)=2tan(α/2)/[1-tan^2(α/2)] +(1-tan(a/2))/(1+tan(a/2))=[1+tan^2(α/2)]/[1-tan^2(α/2)]=1/cosa