tan(π/4+a)=(tanπ/4+tana)/(1-tanπ/4tana)=(1+tana)/(1-tana)=21+tana=2-2tana3tana=1tana=1/3 1/(2sinacosa+cos²a)=(sin²a+cos²a)/(2sinacosa+cos²a)分子分母同时除以cos²a=(tan²a+1)/(2tana+1)=(1/9+1)/(2/3+1)=2/3
