Answer
Cosine is an even function therefore $\cos{(-\theta)}=\cos{\theta}$.
Thus, $\cos{(-45^o)}=\cos{45^o} = \frac{\sqrt2}{2}.$
Work Step by Step
We know that $\cos$ is an even function, which means $f(-\theta)=f(\theta).$
Therefore,
$\cos{(-45^{o})}=\cos{(45^{o})}=\frac{\sqrt2}{2}.$
