Tan to negative one

Why don't you try it on any calculator, including the one here, and see what you get!!.

tan^-1(x) represents the angle that has x for a tangent.

For example, tan^-1(1.732...) = 60^o or any addition (or subtraction) of 180^o of that angle; so, answers of 240^o, 360^o, etc; also, answers of -120^o, -300^o, etc. are also correct.