Without using a calculator, work out, giving your answer in terms of $\displaystyle \pi $ , the value of;

$\displaystyle arctan ( -\frac{1}{\sqrt{3}} ) $

I know how to work out arcsin and arccos, by looking at the x and y values on the unit circle, but how do I work out acrtan? because the way I understand it is that tan = sin/cos which is equal to y/x.

Can someone please explain how to work this out?

thanks.