How to evaluate $\cos(\frac{5\pi}{8})$? I'm  sorry I don't know the way to input pie (3.14) don't have symbol on  my pc

Recall the identity
$$\cos 2\theta=2\cos^2 \theta-1.\tag{1}$$
Let $\theta=\frac{5\pi}{8}$. Then $\cos 2\theta=-\frac{1}{\sqrt{2}}$. To finish, note that $\cos(5\pi/8)$ is negative.  
