<p>okay i will try to explain this the best i can</p>
<p>0 =< x =< 2pi
this means that the value is within the 4 quadrants or 360 degrees (2pi in radians)</p>
<p>cos x = -1/2
cos is negative only in the 2nd and 3rd quadrant
but with -1/2 it has to be in the 2nd (left one up two)
this creates a 30 60 90 triangle with sides of 1, 2, rad3, and an angle x of 60 degrees or 2pi/3
4pi/3 also creates the same angle and thus value in the 3rd quadrant</p>
<p>hope this helps, i know i didnt explain it very well haha</p>