Hello db5vryLook at the attached diagram, showing . The other angle in the diagram whose cosine is also is in the fourth quadrant: it is .

If we allow angles of more than , the next angle whose cosine is 0.5 is ; the next is , and so on.

So if , the possible values of are

To find the values of , then, you'll need to divide each of these by to get:

(Cosine is positive in the first and fourth quadrants, of course.)

Grandad