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.
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.