To find the value of cos 120 degrees, we can utilize the unit circle and trigonometric identities.
First, we can recognize that 120 degrees is in the second quadrant of the unit circle. The reference angle for 120 degrees can be found by subtracting it from 180 degrees:
180° – 120° = 60°
In the second quadrant, the cosine function is negative. The cosine of the reference angle (60 degrees) is known to be:
cos(60°) = 1/2
Since we are in the second quadrant, where cosine values are negative, we have:
cos(120°) = -cos(60°) = -1/2
Thus, the value of cos 120 degrees is:
-1/2