tan(a+π/4）＝2=(tana+tanπ/4)/(1-tanatanπ/4)=(1+tana)/(1-tana)tana=1/3sinacosa-cos^2a=(sinacosa-cos^2a)/(sin^2a+cos^2a) (上下同除以cos^2a)=(tana-1)/(1+tan^2a)=-1/5