Cos(120°) = -1/2
Cosine — the x-coordinate of the corresponding unit-circle point, or adjacent/hypotenuse in a right triangle.
How to derive cos(120°)
120° is in quadrant II (cosine negative). Reference angle = 60°, so cos(120°) = −cos(60°) = −1/2.
Unit-circle context
The angle 120° corresponds to 2π/3 radians and sits Quadrant II on the unit circle.
Its reference angle is 60°, which is why cos(120°) shares its absolute value with cos(60°).
Other trig values at 120°
Related cos values
Frequently asked
- What is the exact value of cos(120°)?
- The exact value is -1/2. Its decimal approximation is -0.5.
- How do you derive cos(120°)?
- 120° is in quadrant II (cosine negative). Reference angle = 60°, so cos(120°) = −cos(60°) = −1/2.
- What is 120° in radians?
- 120° equals 2π/3 radians (multiply degrees by π/180).
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