Final answer:
The value of cos 2π/3 is -1/2, considering the angle is in the second quadrant where cosine values are negative.
Step-by-step explanation:
If θ = 2π/3, to find the value of cos θ, we can refer to the unit circle or use the common values of trigonometric functions at specific angles. The angle of 2π/3 radians corresponds to 120 degrees, which is in the second quadrant of the unit circle. In this quadrant, the cosine values are negative, and since cos(2π/3) is the same as cos(π - π/3), we can use the reference angle of π/3 to find our value. The cosine of π/3 (60 degrees) is 1/2, but because we are in the second quadrant, the cosine will be negative. Therefore, the cosine value for 2π/3 is -1/2.