คำตอบ - Trigonometry

Reflecting the number with respect to 360 degrees.

$$\cos \left(315°\right)=\cos \left(360-45°\right)$$

The period of trigonometric functions is 360 degrees.

$$\cos \left(360-45°\right)=\cos \left(360-45-360°\right)$$

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

$$\cos \left(360-45-360°\right)=\cos \left(-45°\right)$$

Computing the cosine of a negative angle.

$$\cos \left(-45°\right)=\cos \left(45°\right)$$

Computing the cosine of 45 degrees.

$$\cos \left(45°\right)=\frac{\sqrt{2}}{2}$$