Final answer:
The correct answer is option A. The cosine of 2/3π is found to be -1/2 by considering its equivalence to cos(120°) on the unit circle, which lies in the second quadrant where cosine values are negative.
Step-by-step explanation:
The question asks for the cosine of 2/3π in radical form. To find this value, we can refer to the unit circle or use trigonometric identities. The cosine function has a period of 2π, so we know that cosine at 2/3π will be the same as cos(2π - 2/3π) = cos(4/3π).
This is equivalent to cos(120°), which lies in the second quadrant, where the cosine values are negative. The reference angle for this is 60°, whose cosine is 1/2. Since we are in the second quadrant where cosine values are negative, the cosine of 2/3π is -1/2. The correct answer to the question is option A) -1/2.