คำตอบ - Trigonometry

Reflecting the number with respect to 360 degrees.

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

Reflecting the sine function with respect to 180 degrees.

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

Computing the sine of 60 degrees.

$$\sin \left(60°\right)=\frac{\sqrt{3}}{2}$$