A useful number for this problem will be the arctan(3) in the first quadrant. This number is
is approximately 1.249.
The equation can be written as tan(2 θ) = 3. The solution to this equation is θ = (1/2) 1.249 + n π/2
where n is any integer. This can be checked by direct substitution because tan(n π) = 0 for integer n.
The values of n that cause θ to be in the standard interval are n =0, n = 1 , n = 2, n = 3