should be and
I would think about the unit circle. And how cos is the x values. Then try and find the two angles that will get you a value of .5 and divide them by two since we are talking about cos(2x).
Also you have to think about negative values by going clock wise instead of counter clock wise.
The problem here is that we have confused the units of angle.
3.14 or pi is referring to radians
180 degrees is in degrees
3.14 radians = 180 degrees
so 150 is inbetween -180 and 180.
The unit circle in the above post is with degrees units when your question was stated with radians in mind. If you look back to the initial posts those answers are still correct