Reference Triangles for Inverse Trig Functions

We are given basic inverse trig functions and asked to find the angles by using a reference triangle. I have gone to some websites and read up on this and am just not understanding how they are getting the angle.

EXAMPLE 1: Determine the value of tan -1(-1).

Let us look at the reference triangle for the angle a = tan -1 (-1) or tan a = -1.

What angle, a, has a triangle that has side lengths of 1 and -1, and the length of the hypotenuse is the http://faculty.eicc.edu/bwood/ma155s...l/Image335.gif

The angle, a, is - p /4.

I am following everything they do up until the last part where they give the angle. How is that number determined? Thanks for the help!