Question

Find the value of $\cos {315}^{\circ }$

Answer 1

given, $\cos \left({315}^{\circ }\right)$

$=\cos (1\times {360}^{\circ }-{45}^{\circ })$

$=\cos (-{45}^{\circ })$ since $\cos (2n\pi -\theta )=\cos (-\theta )$

$=\cos {45}^{\circ }$ since $\cos (-\theta )=\cos \theta$

$=\frac{1}{\sqrt{2}}$