tan2a=1/3
2tana/[1-(tana)^2]=1/3
1-(tana)^2=6tana
(tana)^2+6tana-1=0
(tana)^2+6tana+9-10=0
(tana+3)^2-10=0
(tana+3-√10)(tana+3+√10)=0
tana=-3+√10或tana=-3-√10