Tan^-1 sqrt 3/3 = ?
let us consult to the exact value table of trigonometry which is attached.
So,
Now its very clear from the table that the value of tangent ratio is sqrt3/3 at 30 degrees.
Therefore,
Tan^-1 sqrt 3/3 = 30 degree.
Now,
we convert the degrees to radians.
30 degrees = 30 x pi/180
= pi/6
which is your exact value.