คำตอบ - Trigonometry

Reflecting the number with respect to 360 degrees.

$$\cos \left(260°\right)=\cos \left(360-100°\right)$$

The period of trigonometric functions is 360 degrees.

$$\cos \left(360-100°\right)=\cos \left(360-100-360°\right)$$

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

$$\cos \left(360-100-360°\right)=\cos \left(-100°\right)$$

Computing the cosine of a negative angle.

$$\cos \left(-100°\right)=\cos \left(100°\right)$$

Reflecting the number with respect to 360 degrees.

$$\cos \left(100°\right)=\cos \left(180-80°\right)$$

Reflecting the cosine function with respect to 180 degrees.

$$\cos \left(180-80°\right)=-\cos \left(80°\right)$$