Evaluate cos(120 degrees )

Problem

cos(120)

Solution

1. Identify the reference angle by finding the distance from the angle to the nearest xaxis.

(θ_ref)=180−120=60

2. Determine the quadrant of the angle 120

90<120<180

Quadrant II

3. Determine the sign of the cosine function in Quadrant II.

cos(θ)<0

4. Apply the reference angle value for cos(60)

cos(60)=1/2

5. Combine the sign and the value to find the final result.

cos(120)=−1/2

Final Answer

cos(120)=−1/2

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