คำตอบ - Trigonometry

Reflecting the number with respect to 360 degrees.

$$\cos \left(120°\right)=\cos \left(180-60°\right)$$

Reflecting the cosine function with respect to 180 degrees.

$$\cos \left(180-60°\right)=-\cos \left(60°\right)$$

Computing the cosine of 60 degrees.

$$-\cos \left(60°\right)=-\frac{1}{2}$$