已知tan2α=1/3
二倍角公式：tan2α=2tanα/[1-(tanα)^2]
即：2tanα/[1-(tanα)^2]=1/3
6tanα=1-(tanα)^2
(tanα)^2+6tanα-1=0
tanα=[-6±√(6^2+4)]/2
tanα=-3±√10
(tanα)1=-3-√10
(tanα)2=(√10)-3
已知tan2α=1/3
二倍角公式：tan2α=2tanα/[1-(tanα)^2]
即：2tanα/[1-(tanα)^2]=1/3
6tanα=1-(tanα)^2
(tanα)^2+6tanα-1=0
tanα=[-6±√(6^2+4)]/2
tanα=-3±√10
(tanα)1=-3-√10
(tanα)2=(√10)-3