#3(1-cos2a) =tana or 3(1- (cos^2a-sin^2a)) =sina/cosa# or
#3(1-cos^2a+sin^2a) = sina/cosa or 3(2sin^2a) =sina/cosa# or
#6sin^2a =sina/cosa or 6sina =1/cosa or 6sina*cosa=1 or 3*sin2a =1 or sin2a =1/3 or 2a = sin^-1 (1/3) ~~19.47^0 or a ~~ 9.74^0#
In #0<2a<2pi# Also # 2a= pi-2a=180- 19.47=160.53^0:. a= 160.53/2=80.27#
Solution: In #0<2a<2pi , a ~~ 9.74^0 ,80.27^0# [Ans]