Trig - Quadratic Function Help

Hi, first time poster, long time reader, Here I am as im stuck on some questions for a quiz. Ive looked in notes and books and still cant figure them out...anyway, here they are. Can anyone give me a clue, how I do it...or even the answers...aha..preferably both!

What angles (in radians) in have a cosine of

1 /2

I know the answer to this is pi/3.

The next question is:

In which quadrant is the other angle whose cosine is

1 / 2

And I know it is Quad 4...

Now...what I dont know is Part C) which is....

Angle not in 1st quadrant:

So how do I find out the angle not in the first quadrant (which is worked out to be Quad 4)?

...............

(I am also stuck on the same question where the angle is pi/6 and need the angle in the 3rd quadrant)