Řešení - Trigonometry

Reflecting the number with respect to 360 degrees.

$$\cos \left(280°\right)=\cos \left(360-80°\right)$$

The period of trigonometric functions is 360 degrees.

$$\cos \left(360-80°\right)=\cos \left(360-80-360°\right)$$

Removing or simplifying the same numbers on the top and bottom of a fraction.

$$\cos \left(360-80-360°\right)=\cos \left(-80°\right)$$

Computing the cosine of a negative angle.

$$\cos \left(-80°\right)=\cos \left(80°\right)$$