Final answer:
The value of x on a unit circle at an angle of 120° is -1/2, because it corresponds to the negative cosine of 60°, which lies in the second quadrant where the cosine values are negative.
Step-by-step explanation:
The student has asked about the value of the x-coordinate on a unit circle when the terminal side of an angle θ intersects the unit circle at an angle of 120°.
In trigonometry, the x-coordinate corresponds to the cosine of the angle, and the y-coordinate corresponds to the sine of the angle. For an angle of 120°, which lies in the second quadrant of the unit circle, the cosine is negative. The cosine of 120° is actually the cosine of 60° taken with a negative sign because 120° is supplementary to 60° (which is 180° - 120° = 60°).
Since the cosine of 60° is ½, the value of x at 120° will be negative ½. Thus, the correct answer to the question would be (a) -1/2.