คำตอบ - Trigonometry

The period of trigonometric functions is 360 degrees.

$$\cos \left(720°\right)=\cos \left(720-360°\right)$$

Subtracting one integer from another.

$$\cos \left(720-360°\right)=\cos \left(360°\right)$$

The period of trigonometric functions is 360 degrees.

$$\cos \left(360°\right)=\cos \left(360-360°\right)$$

Subtracting one integer from another.

$$\cos \left(360-360°\right)=\cos \left(0°\right)$$

Computing the cosine of zero degrees.

$$\cos \left(0°\right)=1$$