The trig functions have periods. That is, they repeat after so long.
Cosine has period of
That is, they it repeats every 2Pi.
i.e.
=1)
and so on
Use this to find the values where it equals cos(x)=-1/2.
Now, enter integer values in for C. 0,1,2,3,.
Try C=0 and we get
Which
If C=1, we get
And so on and so on.
Note the difference between
If you want the interval

, simply use the formula I gave you and root them out.