Which of the following is the set of all values of x?

Question

Which of the following is the set of all values of x such that x is greater than or equal to 0 and less than or equal to 2π and cos x = -1/2?

A. (π/6, 5π/6)
B. (π/3, 5π/6)
C. (π/2, 3π/2)
D. (2π/3, 4π/3)
E. (2π/3, 5π/3)

This question was on one of my practice tests for my ACT. I didn't understand it at all as soon as I read it. So, could you please answer the question and explain to me how it all works? Thank you very much.

Answer 1

cos x = 1/2 when x = π/3

for cos to be negative, x is in quad 2 or 3
so the angle is π =/- π/3 = 2π/3 and 4π/3

Answer 2

The prior answer was good, except that there was a little typo (the first = should be a + instead).