คำตอบ - Trigonometry

The period of trigonometric functions is 360 degrees.

$$\sin \left(420°\right)=\sin \left(420-360°\right)$$

Subtracting one integer from another.

$$\sin \left(420-360°\right)=\sin \left(60°\right)$$

Computing the sine of 60 degrees.

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