tan(π/4＋a)=[tan(π/4)＋tana]/[1－tan(π/4)tana]=(1＋tana)/(1－tana)=1/2解得：tana=－1/3
tan(a+Pi/4)=[tana+tan(Pi/4)]/[1-tana*tan(Pi/4)]=(1+tana)/(1-tana)=1/2, 2+2tana=1-tana, tana=-1/3.
