tan（π/4+a）=[tan(π/4)+tana]/[1-tan(π/4)tana]=(1+tana)/(1-tana)=2tana=1/3sinacosa+cos²a=(sinacosa+cos²a)/(sin^2a+cos^2a) (上下同除以cos^2a)=(tana+1)/(1+tan^2a)=4/3*9/10=6/5