The smallest positive angle in the first quadrant whose tangent is 1 is 45° or
.  You can add 180° or  to this angle as many times as you like.
That will put it in the 3rd quadrant, where the tangent is also positive if
you add an odd number times 180° or , and back into the first
quadrant if you add an even number times 180° or .

Answer: If in degrees, 45° + 180°n
        If in radians, ,
        where n is any integer, positive, negative or zero.   

Edwin