利用诱导公式。
tan(a+π/4)=1/3(tana+tanπ/4)/(1-tanatanπ/4)=1/3(tana+1)/(1-tana)=1/33(tana+1)=1-tana3tana+3=1-tana4tana=-2tana=-1/2
tan（a+π/4）=1/3tan（a+π/4）=(tana+1)/(1-tana)=1/33tana+3=1-tanatana=-1/2
