tan（a+pai/4)=x 用未知数x表示,比较简便
tan（a+b）=tan[(a+pi/4)+(b-pi/4)]
=[tan(a+pi/4)+tan(b-pi/a)]/[1-tan(a+pi/4)*tan(b-pi/a)]
=(x+4)/(1-4x)=5
tan（a+pai/4)=x=1/21
B